Pay with a credit card through Paypal, then you can do a chargeback if all your other options don't work.
I've only done it once and that was with a big, heavy, vintage stereo receiver. It came with one channel out and my stereo fixer said he couldn't get parts for it so it was unfixable.
I told the seller the bad news.
The seller said no refund unless I sent it back at my expense. I said, I'm not paying $35 to send back a worthless piece of junk. He said no refund then. He insisted.
I told him I'd give him time to make up his mind and I'd store if for a month before I trashed it, but I'd have to charge him $2 per day storage. He sort of went ballistic and then quit talking to me. Never heard from him again.
Paypal told me I had to send it back or THEY wouldn't refund my money. I said (again) I'm not paying $35 to send back a piece of junk, especially since this is the SELLER'S problem. They said, the rule is.......you have to send it back in order to get a refund.
I got on the phone with them and said fine, I won't get a refund, I'll do a chargeback on my credit card and get my entire amount back and trash the worthless receiver. They said they could see my point and that was the last I ever heard from them.
The seller, bless his heart, re-charged my credit card for the full amount about a month later. The credit card company automatically rejected it after a few days so that ploy didn't work for him.
I suppose the same thing would work with a knife as long as it's the seller's mistake. I see no reason why I should pay return shipping when it's not my mistake. NO WAY.
The main thing is......you just have to be stubborn and stick to your guns no matter what anybody says.
I've lived my whole life like that......who do they think they're dealing with?