I guess I'd be happy to pay ebay style prices, maybe a smidge more (smaller site and all that). Perhaps around the £1 mark plus extras depending on what extra options were avaialble, or perhaps a percentage of the final auction price (like traditional auction houses).
i dont think the percent option will work as well becuase it will be hard to get the money in that way, so i think the fixed price will be the best way to go