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

Why settle for used. You can get a new Audio Mirror D1 DAC for $500 new on Audiogon (sounds like 3 - 4x the price). I just bought one and am thrilled with it. One note, it is non-up/oversampling so if you're not into that philosophy then you'll need to consider others. Doing my research I have seen great products from Scott Nixon, Bel Canto, MBS, Ack,and others in your price range. Look around and do some research and I'm sure you'll find what you're after.

Pugstub, the world is your oyster in that price range. Musical Fidelity make the A3 24 which sold for $1200 new and can be had for around $550.
EAD made the wonderful 7000 series III that I've seen go for around $450 and sold for almost $3000 when new.

There's one thing for sure, at the $500 level, if you buy it and find that you don't like it, you can likely resell it within a week or two and not lose a penny.

Be bold, read a few threads and take the plunge!

Rooze

the $500 dACk (Version 2) that was just listed a few hours ago probably has little or no competition at that price. I do not know the seller or his particular DAC but I know many people who have had great success with the earlier versions.

audio note

I can tell from having the Birdland Audio Lite dac from the time it came out til now that this dac is great.If you want a smooth sounding dac with lots of detail this is it.Do your self a favor and read the reviews.