It's been suggested before, it'll be suggested again.
My thought on this from way back was to go one better than what FS2 has. In that game, you get a random listing of ads, then wait for it to refresh. If you're looking for something specific, there's a good chance you'll be looking a while before you finally find one listed for sale.
Instead, I had proposed a Global Market, simply go there, and then buy or sell 1/5/10 of any item in your inventory, the prices will auto adjust as sales are made. A database on the TL server would keep track of how many of an item are available in the Global Market. The more of an item, the cheaper it will be, fewer, more valuable. Items are sold TO players for more than items bought FROM players. All sales are to the Global Market, not between players directly.
I was thinking we should actually repurpose the Marketplace building already in the game, possibly make it a Level 6 expansion to open up this function.