I would send him an email stating that it is common practice for fields to sell off their rental equipment as it becomes outdated. Unless WGP looked up the specific serial number against a list of known stolen items, then there is no reason to suspect that this was stolen. And let him know that by winning the auction he is contractually bound to purchase the marker unless he can prove that it was indeed stolen.
If he doesn't like that, then I would do the non-paying report, and leave him bad feedback. Sure he will return fire with bad feedback of his own, but you can add a comment to his post stating the facts. The negative will suck, but others need to be aware of his attempt to back out of a sale. Hopefully you've got enough positive feedback that one negative won't hurt too much. I think eBay gives you the option to challenge negative feedback. I'm sure it's a pain, but it might be an option.