IF and that's a big if, players want to roll, I give them two choices:

1: Group roll, one guy rolls and everyone else "helps". Insert boost die, whatever. Players usually find less this way, but better things.

2: Everyone rolls and everyone finds a few different things. This is a good trick for "letting" players find the McGuffin. Players tend to find more things, but most of lower value.

There's no right or wrong answer here and I don't think even the Devs could tell you otherwise. Choice One represents an organized searching party IMO where the players put their heads together and go hunting for easter eggs in a concerted manner. Looking for one specific thing, or following one specific plan may cause them to overlook things that would have interested them specifically, or maybe just caught their eye. Choice Two represents the party going their own way, looking for what they want personally, but having individually less money and thus potentially overlooking high-value items that may have party use, but be of no use to their own character.

Indeed there are likely times when you want to oscillate between these two types of checks. A Bazaar might be a great place to use Choice Two as the various trinket vendors are coming and going throughout the day. Choice One may be more effective in a more regular emporium-type location where goods and services are consistent and constant.

Alternatively, noone rolls, see below.

As a standard table rule regardless of system, I refuse to roll-play or role-play shopping trips with my players. When searching for an item, I've already determined what the town has (I roll to determine the highest value of items any vendor has), so they don't even have to roll, they can just straight up ask "we want to buy XXXXX" and in turn they'll be told "Sorry, noone has something like that here." And that's my final word on the subject. For major worlds like Coruscant, I can't reasonably say that the entire planet doesn't have what they're looking for, but now it costs them time. It's probable that somesentient somewhere on the planet has what they want, with a population of over 1 trillion, unless the item they want is exceedingly rare, it may be available, but searching a planet of that size takes considerable time. Since I don't want to spend an absurd amount of time on shopping, I turn to the rarity guidelines. For "normal" planet and therefore "normal" rarity, I use the rarity value as the number of days the players will need to take to locate the item in question. Final answer. No amount of checks is allowed to lower this.

So what I'm really getting at is that in a free-roaming game, with sufficient time and enough money, your best answer is to skip the shopping trip roll/role-play and just say "okay, it takes you X days and costs you Y, please update your inventory". In a more time-sensitive game, let your players push the clock if they want to, that my friend is fun . *evil GM smirk followed by some hand rubbing*

However, one thing I've always enforced is that it's "one check a day". I know it may seem a little silly to spend a WHOLE DAY performing a task we back in reality might only take an hour to do, but the goal is really to cut down on the number of rolls made for each in-game day, which can become quite high with certain players. (I have one who really likes role-playing shopping trips, heaven help me I have no idea why, but I try to indulge him from time to time with kooky NPCs and interesting products *coughjunkcough*)

Again: there's no right or wrong answer and many of these things may be situational. The end goal is, IMO: to minimize the amount of real-world time spend on shopping trips. Whichever option is going to accomplish that the quickest is the option I encourage for that specific event. It's not hard to nudge your players in your preferred direction with a little carrot-and-stick action.

I had replied back when this discussion was accidentally double posted. Here's my reply from back then. (just to keep in one place)

I think the intent was to leave the specifics up to how you want to run your game. However, a few points.

• There is precedent in some of the published adventures for having the character with the highest rank in a skill - stealth, for example - to make the roll for the party. This gives players who invest in skills like negotiation and streetwise or talents like Know a Guy or Black Market Contacts, to have their times to excel, even if that comes more often between scenes.
• Many of the skill checks are one check to determine outcome. You can have the players discuss how they are going about their search, how they are each contributing, and add setbacks or boost dice depending on what's happening. You might give the Face character a setback for not knowing anything about a blaster the merc is looking for. Or, if the merc is with him, describing the item directly to Mr. Fence, that can be a boost instead. But, once the roll is made, that is the outcome, period. You can try again later in a different location or situation, but something has to change in the setting before you do.
• You could allow re-tries if you want to. Perhaps add a purple or black for each re-try.
• Skill checks are skill checks. While this may be taking place between scenes, I still do it in session, after the Destiny pool has been rolled, and I have no problem flipping a point to upgrade the difficulty, especially when they are looking for Restricted items.

There's a lot you can do with how the dice come out to interpret what exactly happens. If the check actually rolled a lot of successes, but they were canceled out by failures, you could say "you've covered every inch of that marketplace and talked to every merchant. Nobody has what you're looking for." Maybe they fail, but have advantage. That could mean they find something similar, or a merchant gives them a referral to someone - in another system - that has what they are looking for.

You can use threat to drain them of money, such as paying docking fees or bribes for information, whether or not it turns up the item. Success with threat can also mean that they find a merchant who has the item and knows that nobody else has it, upgrading the merchant's negotiation.

I think the intent was to leave the specifics up to how you want to run your game. However, a few points.

• There is precedent in some of the published adventures for having the character with the highest rank in a skill - stealth, for example - to make the roll for the party. This gives players who invest in skills like negotiation and streetwise or talents like Know a Guy or Black Market Contacts, to have their times to excel, even if that comes more often between scenes.
• Many of the skill checks are one check to determine outcome. You can have the players discuss how they are going about their search, how they are each contributing, and add setbacks or boost dice depending on what's happening. You might give the Face character a setback for not knowing anything about a blaster the merc is looking for. Or, if the merc is with him, describing the item directly to Mr. Fence, that can be a boost instead. But, once the roll is made, that is the outcome, period. You can try again later in a different location or situation, but something has to change in the setting before you do.
• You could allow re-tries if you want to. Perhaps add a purple or black for each re-try.
• Skill checks are skill checks. While this may be taking place between scenes, I still do it in session, after the Destiny pool has been rolled, and I have no problem flipping a point to upgrade the difficulty, especially when they are looking for Restricted items.

There's a lot you can do with how the dice come out to interpret what exactly happens. If the check actually rolled a lot of successes, but they were canceled out by failures, you could say "you've covered every inch of that marketplace and talked to every merchant. Nobody has what you're looking for." Maybe they fail, but have advantage. That could mean they find something similar, or a merchant gives them a referral to someone - in another system - that has what they are looking for.

You can use threat to drain them of money, such as paying docking fees or bribes for information, whether or not it turns up the item. Success with threat can also mean that they find a merchant who has the item and knows that nobody else has it, upgrading the merchant's negotiation.