I agree penny, I always take a deep breath and walk away for half an hour to calm down and then return to the keyboard. Personally I believe that the cheeky offers ($10.00 on a $35.00) are really teasers to draw/suss out the amount that we are prepared to accept.

Regardless I always reply politely and thank them for their offer, then give them a counter-offer of the lowest $ figure we will accept. Probably 75-80% of the time they accept our counter offer. So I don't write these offers off, just treat them like potential customers and who knows we could have a sale.

One caveat here is that sometimes they return with a counter to my counter offer, but i don't move. I reitereate the original offer, and reply again politely that this is the price. Very few go this far, mostly they either disappear into the ether, or they accept my original  offer. Works for us!