Once a month a Farmers Market appears in the wild .... er Providence. Lasts for 72hrs Friday Midday to Monday Midday which pretty much gives everyone an opportunity to at least visit it, the day before the markets Organiser appears.
Players can visit the Organiser and place up to 2 items (the market should have a limit on the maximum amount of spaces - such as 3 stalls worth) for sale at whatever price they set. These items will appear on the stalls on a first come first served basis, so once an item is sold the next on the list is displayed (probably have to re-click the stall to view it) with the Organiser being able to hold a max of 50 items in reserve at any one time.
Only players without a stalls can play items, although honestly i'd be up for accounts without a stall or players without the mercantilism skill.
At the end of the market any silver made, or items unsold are mailed back to the player. Taxes are obviously applied (10% perhaps).
Towns like Providence were nasty, disease ridden and dangerous places but people still flocked to the them, perhaps we should have a few more in-game reasons to be good little flockers?