You should not have been able to exchange details (address etc.) before the buyer paid and payment on collection is not actually allowed.

Considering doing an 'off Ebay' deal may put your account in jeopardy.

You are considering cancelling a completed sale - you can do this by going to 'orders' and clicking on 'view order details' - you don't need to be in seller hub to do this.

Then, you can message the lower bidder.

The original buyer would still be able to leave you negative feedback and could still turn up to collect the trailer.