I haven't received emails of that specific kind 
before, but I've been in several 
online.. "spats" in my lifetime.  I think the 
best thing to do is ignore the email and follow 
Star Phoenix's recommendations if this was 
through eBay.

I find that writing back to argue with people 
only makes me worry all day about what kind of 
response I'll get, and I get a little nervous 
each time I get an incoming email.  It's stress 
I just don't need.  So now I just ignore their 
emails, block their email address, and if they 
try to be cute and send it from a different 
email address, I delete it without reading it 
and block that address as well.

Sometimes its very, very tempting to give them a 
piece of your mind, but it is rarely rewarding 
in the end and it's usually better to just let 
it go.