ok it was Zap_Rosdower of gamefaqs who first posted it
i now confirm it works in other shops, but didnt' test every one--i assume it applies to all, though -_-
it's very simple. equip stuff you want to sell on ppl, and go to the shop, sell their equipped items. the sale price for a Gold Medal is 2000 potch. if you sell it from your Caravan, you will get 2000 like it tells you. however, if you sell an equipped Gold Medal, it still says 2000 but you get 4000!