Oh dear! Since you are new let me cover the basics just in case. In trading we typically use "currencies" such as 999's, 3333's, 4999's, 5999's, and 6666's. Items are identifed by their sell back rate, so an item that sells back for 999 coins is known as a 999. I'm not good with prices either, but my guess for the current price of Hideeni costumes would be 10-15 5999's. You can offer and if people think your price is too high, you can always change it. Also, if you would like another Hideeni costume in exchange for the one you have, that would be fine. If you are interested in buying rares, you can always trade your Hideeni costume for rares instead of asking for the normal currecy items. This can be trickier though because in this case you would also need to know what the rares are going for. The best people to ask about the pricing of rares would be the mods on this site. I hope I didn't confuse you.