When you can't find what you're looking for in a shop, you'll ask the clerk. But what if the same thing happens to you at a fleamarket?
The solution: special fleamarket guides who help you find what you're looking for. There could be specially trained people who are in charge of a certain stretch of the fleamarket and can tell you what's going on. If it's too much to remember (possibly!), the stands could also enter their stuff into a database, so everybody would know that e.g. stand 37 sells the hockey equipment you've been desperately looking for.
Reward: Perhaps some free fleamarket shopping?