I think that demanding $50 (firm) is a little bit messed up. Would you really turn someone down if they had a 1,000G tank and offered you $10? You should try to find a good home for it even if that means giving it away. I'm not trying to be mean, but you made a mistake buying the fish, so chalk it up as a loss and learn from the mistake. But considering it's 13" and in a 10 gallon, I think the best move you can make is to take it back to your LFS ASAP. There are more potential buyers at a LFS than you are able to reach through craigslist ads or anything like that.
Good luck!
Good luck!
Though I often disagree with people for flaming in this case I don't think anyone really did until the OP got defensive. Also I don't often agree with the size tanks many suggest on here. I believe depending on the fish it's personality and how active it is greatly affects the size of the tank they can be comfortably kept in. For instance Gulper cats get pretty big about 11 inches but don't really need as much room as some other fish and can be kept in relatively small tanks ie: a 55 gallon. While a Cichlid like say a Loisellei prefer to actively cruise a territory and therefore need a bigger tank despite being of similar size. So I'm not a person who jumps down people's throats for tank size. How ever in this case a 10 gallon is just cruel and as many have said you shouldn't be holding out for a price to make up the loss that your responsible for. If you've read any of the stickies you'd know even a 180 is too small for a RTC. They get long and wide a double whammy and on top of that they're not a typical hide and stalk catfish predator. They're open water cruiser's who need lots of space to move. Plus as it was stated by onetouch2many you should have learned the first time with the TSN.