Firstly, trying to resolve the matter via eBay messages is time-consuming and haphazard at best, so go through Advanced Search at the top of this page and pull his contact details. You'll probably find that once you're in a position to contact him by phone any time you like, he will prove to be far more amenable to a rapid solution, particularly if you explain to him exactly what you said above about the inspection and the fact that the owners want the trailer gone from their rented premises immediately.
You could offer him a refund minus the listing and final value fees that you had to pay eBay and which are now unrecoverable, plus storage fees at the rate of $10 per week for the 3 months in which he's failed to collect it (assuming there's anything left to refund after that) then try relisting it in the hope of a quick sale and disposal, or you could simply tell him that you will be moving the trailer out onto the street and he'd better come and pick it up before somebody steals it.
There are also certain state laws pertaining to uncollected goods which you may want to google so that you know exactly where you stand in relation to refunding moneys and/or disposing of the goods, and make sure that whatever you ultimately do, you keep within those guidelines, but obviously the first thing you need to do is to talk to the buyer and ask him what his intentions are.
