I would reply and say you are unable to send them an offer.
Tell them you are happy for them to make an offer as permitted by the listing.
If they won't do that, don't deal with them.
This sale needs to follow Ebay procedures so you and the buyer get the benefit of the authenticity guarantee
"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)
