You advertised a clean amulet to sell, which at the time was correct.
The amount of time it took the buyer to decide to get it is of no consequence, you could have sold it to anybody else in that time, it was not off the market.
You lotto'd it in this window and it failed but still attempted to sell as clean, that's the first issue.
Buyer eventually noticed that what he bought was not sold as advertised.
Buyer contacted a mod to resolve the situation, you decided to give a partial refund, not following the mods instructions.
At this point, I'd imagine a slap on the wrist if you returned the TG in full, owned it and apologized. The amount you have and the amount the buyer has is of no consequence.
However, you refused because you wanted to keep the sale for whatever reason, when everything that went wrong so far could be traced back to you advertising an item incorrectly.
Whether or not he noticed after fifteen minutes, I've bought items and not personally checked them roll for roll before. I should have and maybe I've been scammed in the past, but it's not something I've ever looked for or noticed.
You are a scammer because you advertised an item falsely, sold the item falsely, were mad when you were caught selling the item falsely, and most importantly, refused to make the situation correct.