Welcome to the site, fiche! You can work out the value of creatu using the Search Pets feature (see the navigation on your left.) Search for the lowest price of the species and colour of the creatu you want to sell, and price around that. Names can add value too - popular names like 'Alice, Bunny, Fred and Pikachu' will sell better than names like 'Lorayia, wildcat66, asdjkgfe and AliceSmitch' simple beacuse they're popular and people want or collect hem. so if you have a really good, real name, you can add some tu to your price. Some names sell for millions because they're so popular.
Good luck!