I would message via Ebay messages and tell them that they can ask any questions they have through Ebay, by replying to YOUR message.
If they don't, I would decline the offer and block them.
Are they a UK buyer? The authenticity guarantee protects you as well as the buyer but doesn't apply to items sent overseas.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)