
Your best bet would probably be to not respond in kind. It would just make you look bad. Collect all the evidence you can find and your emails and forward it to eBay's abuse department (or what ever area they have set up to handle that). Anyone that behaves that suspiciously and with that sort of childish retaliation should be banned.
You've got 90 days to leave bad feedback in return so you have time to see if eBay can/will do anything about this.