Ebay won't always side with the buyer!! I've only had it happen 4 times and they sided with me every time!!!! I will give Ebay credit for that. Sometimes, it's not easy and you may have to make a couple phone calls to Ebay.
"Buyers remorse" is not a reason to return an item.....Unless!!! You say in your listing that you accept returns. Or how you have that worded. I have in my listings that "I do not accept returns unless an error on my part. So please look at the pictures! That is part of my description since condition is subjective, if you need more pictures or have any questions please message me! I'll be happy to answer any questions...." or something like that.
I then choose if I'm going to accept a return or not. Since you have the message, Because Ebay does look at those, so be careful what you say or promise in messaging, They will see it's buyers remorse. What is written in your above post is certainly "buyers remorse." But...That all comes down to your return policy too!! Ebay will also remove negative feedback if the buyer leaves it.
One was with a pocket watch. The guy was claiming "not as described" So I kept the messages going, he changed his story 3 times before accusing me of scamming. I called Ebay, they looked at everything, and sided with me. Because in one message he was claiming he found one cheaper....And I should give partial refund......Then he kept sending me multiple messages saying "Please". I'd reply with "NO". I'll bet he sent me 30 messages trying all different routes.

I probably would have taken the return for him, but I was dead broke, needed diapers, and accepted his low ball offer of something like $275 on a $500 watch.I didn't even want to sell the watch to begin with. He even tried the "you over charged for the item", I'd send him completed sales listings. The only reason I played his game was because I knew Ebay would side with me and I just let him dig a deeper hole.
With your item I'd do whatever I could to not accept the return. You may not get the same necklace back, then it's real hard to prove who did what. That's what happens with watches since parts are so hard to find!! Parts get swapped, and the watch gets sent back. Usually, you would never be able to prove this happened.
added,,,,
I take that back,,,I forgot about one...I had a lady return software, that claimed it didn't work. Ebay told me they were sorry but there was nothing they could do since we couldn't prove if it was good or not, easily. And I did give the refund because I made very little on it and it would have cost more for return shipping. I issued the refund and told her to throw it away, I didn't want it back if it didn't work. That's the only one. And Software,games, dvd's are a hard category to prove anything and you make very little when said and done. They did say they would watch that buyer for repeat returns.