According to the RT rules, players must first make an availability test based on population to see if there item they want is available before they even roll to see if they can acquire it. (Page 111)

I'm allowing players a single acquisition roll per character at the beginning/starting of an adventure to see acquire something. My question is who would make the commerce/inquiry check? The Rogue Trader or the individual searching for the item? Could the Rogue Trader use his class ability (+10% to any test) to help this check? If so, would he then be able to use it on the player's acquisition roll?

If this is their first mission, I assume you know that characters get a free single item with an overall acquisition modifier of zero?

Otherwise, I'd say it's down to the person who wants the item to search for it, unless they all give the Rogue Trader/Seneschal a shopping list. I'd also probably say yes to the Rogue Trader's +10 modifier, but only if he is with them whilst they search.