I really don’t think any kind of revenge is a good idea. It will only weaken your case.
Also, refuse to take delivery of any unexpected packages that arrive in the next week or so - not because it could be something nasty, but because the buyer might just send you the shoe tree, then use the proof of delivery from that to press through his refund claim.
Also, let’s just consider the statement: “I was warned about you by someone who dealt with you before” - what does this really mean?
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Someone randomly contacted him and said something about you before he even bid on the item? Not likely.
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Someone saw him win the item, and then contacted him? - would mean you have a stalker, which would be evidenced, I think, by other buyers letting you know everything went well, but some nutball emailed them.
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The statement about the warning is an outright lie? Quite possible.
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The ‘warning’ took place on some forum - possibly an eBay discussion board - where the buyer has asked a question and the transaction is being discussed. Quite likely, I think.
I don’t have sufficient details to begin searching for that discussion - but you do - the item number, the buyer’s ID and your ID (as well as terms from the description of the item). See if you can find that discussion and see if what’s been said there matches up with what you’ve been told so far…