There's an expctation to refund because those are the rules under eBay's 30 day MBG. On collection a buyer can come and fully inspect the item.
The buyer has come and inspected and perhaps seen the dents and rust patch are bigger than expected? Whatever the reason they have walked away , you have the item and the money.
You cannot force them to take the item, annoying as it may be, you do need to refund.
Cancel the order citing Buyer Requests to cancel. This fully refunds them and your seller performance is not affected.
You can then relist the item if you wish.
@hbk.ant