If you are in the United States, you would likely be considered a money transmitter. There was a recent ruling by FinCEN:
https://bitcointalk.org/index.php?topic=154686.0There are quite a few others on the same topic. Here is a random web page on the topic:
http://bitcoinmagazine.com/fincen-bitcoin-users-not-regulated-exchanges-are/Before anyone takes what I say too seriously, I am not a lawyer, but I have a related degree, so my analysis comes through my particular lens. I think it could be argued that if you are acting as an agent to convert money, or acting to 'exchange' monetary forms, you would need to be licensed. If you do enough reading on the FinCEN rulings, and take some time with reputable resources to understand exactly what the ruling means, then I think you should have a pretty good feel of what is legal or not. I think you may come to the same conclusion as I.
In reality, I doubt you need to worry about it. The only real threat that I can think of is some under-cover Treasury Official trying to buy Bitcoins for the sole purpose of prosecuting someone. In order for that to happen, someone would really need to have a bug up their ass over Bitcoins. I just don't seeing it happen.
