I think a "Where can I get this from?" section could be a good idea.
Be strict about the title of the thread, to make it an easily searchable archive, so put something like,"Half inch Copper washers", so if someone looks through, its easy to see.
Different users will have used different places to obtain goods from. If its something like electronics goods, then a quick search on E bay will show up the options, but peeps who have used a good, or bad supplyer could comment.
Be strict about moderation, if a supplyer said three day delivery, then took three weeks, state the fact and leave it at that. Make sure a supplyer doesn't get into a slanging match with a disgruntled cutstomer, other wise there would be potential for threats of legal action. maybe make negative comments about supplyers PM only, keep them off the board.
If a supplyer can supply. limit them to one comment per thread, don't let them dominate the thread.
The date is already at the top of the post, so thats not an issue. To avoid a Chinese auction, make the prices PM only, if the buyer wants to post up how much they have paid after, then thats up to them.
Keep it on the forum, not the wiki, its basically just a sub forum, and not really that far away from what happens anyway, it will just be putting it all in one place.
Make it questions only, no on spec posting of other stuff seen for sale.
Good rules and moderation will be the key to stop it getting out of hand.
A few scattered thoughts.