From PsyHelp
The Flea Market is a place to find items that other residents are selling. You can use the "item search" feature to search all the stores for an item you want.
With "store listing" you are linked to the Directory where users with an open store are marked with a 
sign. Click on it to go directly to this users store.
When you search for an item, the results are sorted with the cheapest option from each store listed first. Be careful, however, if you search for a word that multiple items might have in their name. For example, if you search for the word "pawn", even if a particular store has both Black Pawns and White Pawns, only the cheapest offer in a given store will be listed. On the other hand, this may not bother you if you're searching for "egg" and you turn up both Eggs and Speckled Eggs.
To sell items through the Flea Market, move the items to and price them in My Store. See the Newbie Guide for more information.