If you have already paid the payment should be showing in the sellers eBay account and will be released 24 hours after collection generally
If you give him the code before actually collecting he could refuse you and the code is proof you collected-not a good idea.
If he wants cash on collection,not allowed on a normal listing depends what category the trailer was listed in,vehicles category cash is fine,he could relist in vehicles if it wasn't and then you could pay when collecting provided trailer meets your expectations.