Sorry, but now you're just splitting hairs.

Technically, it could be said that you sold them back to the person you bought them off.

Either way, you sold these tickets to the buyer.

Then you got a better offer elsewhere.

Despite your legal and ethical obligations to supply what the buyer bought from you

You renegged on the deal, to your benefit.

You broke the contract via non performance, and I am guessing that the buyer wasn't particularly happy about it and the cancellation wasn't mutual, but rather a case of "suck it up, I ain't sendin' 'em to you, so take the refund and get over it"

......................

Maybe next time, wait until you are sure that you want to sell something, before you do, or at least list it at a price you are prepared to accept for it.

With the details available here, I don't blame the buyer for being miffed