I would get him to bid £500 on the auction, and then if he is the highest bidder, and you want to end the auction early to sell to the highest bidder, you can. You will then know if the current high bidder has bid £500+.
To be honest, without searching through completed items, I don't think yours will go for top whack. Mainly because your feedback is only 1. Although this doesn't matter as much when you are collecting in person, some people will take it into account. You have also listed some faults, which takes down its value. How old is the trailer? Somebody could intimate that it is two years old from the listing. I would add a note to say exactly how old it is.
Don't let anybody pay by Paypal whatever you do. Only accept cash.
It is not against the rules to end the auction to sell to the highest bidder via Ebay. You can also do this in the last 12 hours.
From ebay:
It is acceptable for sellers to end a listing early and sell an item to the highest bidder at their current bid price. Bidders are permitted to contact sellers with requests to end a listing early for this reason; however, sellers are under no obligation to do so.
If you deal outside of Ebay with this buyer, and sell it privately, you will save yourself £23 in Ebay fees. You're not meant to do that though.