Why not remove the player stalls and add a building called Auction House.. have fee for selling items and let players trade there items there and if the items dont sell they get mailed back to the player at post office..
Yes sounds like rip off from WoW but hay it works, only one problem i see with this is being pked by the auction house which they need add some type of guarded area. Maybe add some NPC guards if player does a crime. The guard reacts and attacks them with crime flag on..