There is no best offer, so it is just between you and the buyer. They cannot accept unless you amend the price and even then someone else could jump in.
Add Best Offer to the listing and do it the easy way.
I can only assume you have been making offers through messages as there is no 'Best Offer' option on the item number you supplied, in which case, the link I gave is irrelevant.
Simply sort it out with your seller through messages.
