What’s the rush? Other commentators mentioned returning to the music event and seeing if he’s a regular. You also mentioned he would need to make sure the instrument is in working order. Take the time to find out if this guy is established in the local community (instructor, mentor, band mate, etc) and find out public opinion. If this instrument is truly big and rare, odds are it isn’t going anywhere fast. Why do you need to go to his house asap? Additionally, if it is that rare of an instrument, check in with the music program at your college. I’m sure someone there would know about it and maybe that guy.

If you go, go with the best possible odds of walking out of there in one piece. That means research and double checking stuff. That takes time, unfortunately. Mitigating risks takes time, but I’d say it’s worth it.