The seller cannot mark it as paid as you both have made an off eBay sale.
Payments after purchase has to be made at eBay checkout, cash on collection / payment on collection, is not allowed.
At eBay checkout the buyer receives a 6 digit QR code, and this when handed to the seller to scan, is the only proof the item was collected and paid for , through the correct channels.
Nothing either of you can do to now, to pay correctly, or to mark as received / collected. Just hope that eBay don't pick up, that payment was not made correctly.
If the seller has a shop, then they are a business and really should have known eBay's rules on payments, both accounts can face restrictions / bans if found out.
@rustybarrels