I just ignore people who put in offers via message on eBay rather than block them, but you just have to accept that s/he is doing it their way, and their way is likely a response to previous experience of other people did the same thing, in my case I dealt with a guy who contacted me via eBay via the same approach as the OP here, agreed the sale and price, then started to haggle, then when I refused to move on price became very hostile and aggressive and threatened to report me to eBay for dealing outside of eBay!!!!!!!!!
Either way, personally I'd be looking for a similar unit elsewhere, as strange behaviour on the part of a seller before you've even bought the item doesn't bode well for when you do buy it and there's a problem