$500 is a pretty standard starting offer, it shows genuine interest in a decent name which (assuming it is a genuine offer) in my experience. If it was me with this domain, I'd counter with mid xxxx.
It just depends on what you feel happy with yourself. I generally wouldn't accept an initial offer, if it's around what I'm looking for, I'd probably counter with $750 and take it from there. There is always the danger of the buyer thinking a name isn't worth it if you just accept the first offer imo.