I wouldnt pay 1 gp more for a signed item, but thats just me... One way to prevent market crash would be that only the maker could sell such item and just once,it would be like zezima's rune longsword (1), and after the person buy, it would become untradeable. Another thing to add could be a middle man in the trade, like the estate agent, every person that would want to sell, would have to sell to a special shop, and the prices depending on the shop stock of the items of that person would be different. That should hold off the price manipulation of the popular players.