You should definitely tell a mod. You might be able to get your bells back.
How do you report somebody? I sold somebody a code and they never paid me
I'm pretty sure you can just look at the bottom of the home page of tbt, and find the "who's online" spot. Then just look for a mod that is online, and tell them the problem. Mods names are usually highlighted in red or blue.
(this info is semi- accurate. I haven't ever reported a problem, but I think this is how you do it)
Post his username to give warning to other members.
You can also report them to Nintendo to ban his account.