I would recommend treading lightly here and not jumping to conclusions. For all we know, he and some of his friends ride/work on/race bikes and they were doing what any of us might be doing with bikes in a garage. That said, it is, of course, possible that you witnessed some kind of chop operation.
At this point, I would suggest observing and gathering more info. Try to develop a profile on the guy, maybe check to see if he has any kind of record, find out where he works/what he does and see if he lives beyond his means, etc. If an opportunity presents itself, peek inside the garage. Also, if he's sellin', he must have buyers and they can probably be found. What I would NOT do, is let him get wind that you are raising an eyebrow in his direction, b/c if he is a criminal you DON'T want to give him a reason to direct his attention to you and yours when/if he does get busted.
Bottom line, learn what you can on your own. If your suspicions continue, hire a private investigator. If that bears fruit, bring in the cops. But above all else, remain anonymous to the bad guys.
Best of luck!
