I assumed he emailed you through eBay (i.e it came up in My Messages).
If he did, that side of it's OK. eBay do provide an option called 'Best Offer.' If you've selected that option, he's perfectly within his rights to approach you.
If you haven't, I would probably leave it alone, but on the facts you've given, I can't see too much wrong with his request. Follow your instinct.