I just started a rogue trader campaign and I've a question for you: when a player try to buy a new item (a weapon, an armor, etc...) which test should he do?

The one described on page 111 (inquiry/commerce +- avaibility) or a normal profit factor test? Or both? Me and the master are going crazy trying to understand this...

The chart on 111 is to figure out if the item exists where your looking . A lot of GMs just fiat this part of the check, or at least make sweeping acceptances depending on the nature of the world (such as all imperial technology is available on the Lathes). If this test is used, succeeding doesn't grant your Rogue Trader the item, it only means they've found someone who owns one.

The Profit Factor test is used to determine if you are able to acquire and arrange the supply & maintenance of the item you have previously found. This is generally the more important roll, and the one actually referred to as an Acquisition Test .

Also, see "How to Aquire Items" further down in this forum.

Ok thank you.

So the inquiry/commerce test is not strictly necessary and can be skipped to speed up the game?

It's strictly up to the GM, of course, as skipping it amounts to an application of Rule 0, and/or house rule, but I known many do often skip that step unless time pressure is involved, or the player is looking for the kind of item that might be present but is neither a sure thing (Like a laspistol on Sentilla) or impossible (like an Auspex on a feral world).

I know GMs who have skipped Acquisition at times to, particularly for single common (or scarce) items, just because it felt like the kind of thing a Rogue Trader shouldn't have difficulty picking up. As always, rolls are up to the GM.

Ok thank you.

So the inquiry/commerce test is not strictly necessary and can be skipped to speed up the game?

By RAW, no, but as Quicksilver says, a lot of GM:s do. Rule 0 is the rule saying that the GM has final say and can change anything as he sees fit.

For reference, a game of similar scope, Black Crusade, completely does away with these rules and say that it's up to the GM whether something is available or not.

Ok thank you.

So the inquiry/commerce test is not strictly necessary and can be skipped to speed up the game?

I have often told my players to skip it - they weren't trying to covertly buy a bolt pistol without telling anyone, but rather putting out a contract for enough bolters that whatever company won the bid would be set for years.