I'm hoping that "doesn't turn over" means that the engine doesn't start and not that the motor is seized. I called the phone number listed in the add and left a message, so we'll see what the story is if I get a call back.
This is going to be my first old bike. I currently own an '09 Yamaha R6, and have owned an '04 Honda CBR600RR, but I really like the CB750 mainly because of simplicity, availability of parts, and customizing options. I'm really looking for a project, so I'm not looking for a running motor. But, I was hoping to at least get a motor that turns over and is not seized. However, if the motor is seized, worst case, what might it take to get it back to running? Ideally, I'd like to keep most of the motor. I'd like to avoid major service that I wouldn't be able to do myself (i.e. cylinder boring, complete cylinder head rebuild). I am prepared to open the engine to remove carbon deposits, replace all gaskets, cylinder honing, valve lapping, piston ring replacement, and other things I can do myself.
As far as price goes, I haven't been able to find anything as low as $500, except 1 add for $50, which gets you a '71 CB750K frame, seized engine, trans, and wheels. Here are a couple of pics of the '76 for $500 that I've been talking about. I'm not sure how much you are able to tell from these, but it's better than no pics. Plus, I'm still waiting for the owner to get back to my questions.
Thanks for the advice so far guys. Keep it coming.