Click on their message, find their highlighted username ID then on the right you will see Contact Member.
Send a message through there, but make it very, very clear they're to buy from your eBay listing, that you're not contacting them, for an off eBay sale.
Something like.... ' If you would like to send your original offer through for the ***** item, again through my eBay listing, I'm happy to accept that offer. '
@woodbinesno.1surfer
