If he is any sort of reasonable person he will accept that you bought it in error and your offer to pay his relisting fees is fair. He has nothing to gain by pursuing you for payment.

On the other hand if he so choses, the worse he can do is give you an unpaid item strike, but that is no means certain if you put give Ebay all the facts when asked. This sort of thing happens all the time and as a trader you shrug it off and carry on because you it is not worth the effort trying to enforce payment.