This is what I would want. I'm not an accountant, just thinking aloud
* I would not bypass the ebay process. Bypassing paypal ... sure. But don't go around Ebay. Ebay provides some level of proof you bought it from him. It provides some sort of trail if this is ever found to be a scam or stolen or ...
* I would want a real receipt/bill of sale... printing off the ebay invoice is a start. But some hand written thing on a blank sheet of paper with no letterhead... no. (Yes, letterhead is easy to fake, but a scrap of paper with hand scribbles just screams "run away".
* For that price, seller would also have to have a pretty impeccable rating on Ebay.
I worked for a very large corporation in the security department... and you'd be amazed at the sort of stuff that was stolen/sold on Ebay. I'm talking extremely expensive, highly technical equipment where there are only a few hundred serial numbers total (and every number has a very well known, large corporate buyer associated with it.) I'm not saying to avoid this. I'm saying to tread lightly.