That's not what Karl said in his first message - he wants to go around and check it so "that I can ensure it is a genuine sale and also check it is perfect as desribed." The time for checking and deciding whether it is genuine/perfect etc was before the auction ended. Karl won the auction. He has an obligation to pay. The seller didn't say that he DID accept callers so the deal is winning bid + postage.

Put the boot on the other foot. What if you'd paid to sell an item on Ebay and some guy won it (so you incurred the further Ebay costs) but the guy then said he wanted to come round and have a look before deciding whether to buy it. Would you not think that was out of order? I would.