1. A seller can decide not to send, no matter if you have paid or not. He can easily refund and cancel the sale. Paying does not protect you.
2. The fact that he stated he is aware of stories that people swap out /return fakes means if you do return them, he could claim that you returned fakes, which could make it harder for you to claim "not as described."
3. He can claim he took them to a jeweler to be authenticated, but will he prove that he did so?
4. Sorry but absent paperwork (original sale receipt) showing the clasp originally had an 18K diamond clasp, you only have his word. If he doesn't have the clasp how does he know what it was? If he has it why would he not include it in the listing?
I would not pay yet. Let him take them to a jeweler and let him show you the letter from the jeweler, on letterhead, that they are real. I don't think he will do it. An appraisal would cost him money.
Are you prepared to risk losing your purchase money? If not, don't pay yet.
*off to cut my hair, will come back to thread later.*