If that were the case, why approach a pet store worker and solicit their business instead of posting about it on Craigslist? It's the lack of a trail as much as it is the price that makes it seem sketchy. He could have easily moved those animals in the space of a week at a higher price via Craigslist or some similar listing, yet he chose in person and tried to pawn them off as quickly as possible? Also, if they belonged to the guy, he would have been savvy as to their relative worth and would have likely wanted to make more money off of them. As much as we would like to believe it was just a sweet deal, the likelihood given the situation seems low.
I know if it happened to me though that I would want someone to report it so that I might get the chance to have my animals returned to me. The legality is not so much in question at this point as the ethics are. As mentioned previously, I'd be looking around for lost/stolen ads just to be sure. If someone is missing their animals, they would probably exhaust every possible route in their search for them.