What is the best DAC to buy used in the $500 range

First, decide on what features you need, volume, toslink, coax, multiple inputs, hdcd or....
Secondly, do you want tubes or solid state?
Third, do you like warmer sound, analytical or ???
Fourth, do you need 24/96 or will a great 20 bit unit work?
Finally, looks: Do you need it to match your system?

The Audio Mirror D1 DAC is awesome. I'm glad I bougt it and am considering selling my Nixon and buying another one.

You'll rarely see a Scott Nixon tubedac for sale and there's alot of raves about them. New ones are around your price. I've not heard it but several people I respect have told me it's outstanding. Definitely worth checking out.