The more details I hear, the more I think your buyer simply changed her mind (because it didn't fit, because she saw something else she liked more, because she realised it was more "worn" than she thought, because it was the third day of the rising moon, ...).
If I simply wanted to avoid the "scandal", I would have offered you your money back. Period.
The fact that he offered you a "fair purchase" value is to me a sign of his good faith: you are a valued customer, you no longer want an item that you bought but that is perfectly OK, and you want cash. Fine, no problem: here's how much I am offering - I know that I can resell it at a (small?) profit later, and I don't want you to feel mistreated. If he had any doubts that it was as described, he would have either stalled (if it was a major amount) or "gagged" you with a full refund.
Incidentally, on period pieces, marking can be all over the place - and each nation has its own standard for marking. In France, after about 1900, the gold mark is an eagle's head, and platinum is a dog's head - no numbers whatsoever, and they are tiny and usually poorly struck. I have a pair of Cartier cufflinks that have Italian and French hallmarks, and for good measure an 18k stamp, but I also have several other pieces that are undoubtedly in gold or platinum and have no marks whatsoever.