Extra Fees! You may be living with your costumes for some time. I do my research, but sometimes there is no clue to the going rate for an item and it is merely a question to see what you actually want for the item.
People ask me that all the time. MM you.... geesh I would hate a fee to be attached to that. If you want to add a fee, then when the unfortunate player asks you what you want for an item, add it. Advertising your fee just turns people off and they won't really want to purchase from you, your shop, anything.
When I ask what the player wants, I don't want to lowball them, I merely want to know what they want for the item. I like to play Marapets fair.