I have a 5770 that appears to perform at the same level or better for the power requirements than a 7770 and I paid $35 for it. That might explain the lack of interest.
Can you share your MH/s and Watt numbers for that 5770, because I ran a 7770 at 210MH/s, and it was amazing at how efficient those 7xxx cards are compared to the 5xxx cards.
And I agree with tacotime. Newegg has a pretty large selection of 7770s between 100 and 125. I'd say 75-80 is what you're looking at for a used GPU.