If you have one item and you accept an Offer, then another Offer comes in, you just refuse the second Offer
With any Offer the seller can accept, ignore, or refuse.
It would be good customer relations to explain the reason for the refusal to the second customer.
If the second customer was buying at full price, an item that had already been sold on Best Offer, I don't know what would happen. It's likely that eBay was just slow on updating but also likely that eBay would refuse any responsibility. Sigh.