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

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.

I was in the same situation 6 months ago and my research lead me to the Bel Canto DAC rev 1.1 - the .1 is an power supply upgrade from the original. The BC DAC retails for 1300.00 and they can be acquired on A'gon used for 500-600 dollars. Do a google seach a read the reviews -the BC is an excellent performer

NOTE....which ever DAC you settle on be sure to pair it with a high performance digital cable. When I initially purchased my DAC, I bought a well reviewed budget digital cable and at the time though it was a good performer...that was before I auditioned a Transparent digial reference cable -the difference was night and day. I never would have imagined that swapping out 1 cable would make such a difference, but it does, at least with my setup the diferrence was audible.

Good luck!