Used DACs for sale - help me choose

IMHO,  anything spent above $300 new for a dac these days and you reach the law of diminishing returns.  For instance,  a Schiit Modius $229,  or ifi Zen Dac Signature I or 2 $249 - $300 are such high value products, that it pays to listen to them before you go down the dac rabbit hole.  If you want a better dac you can always spend more. But at least give these a try.  😊

If you are going to buy used, there is the likelihood the model will be out of the warranty period.

When selecting models have a investigation to learn if there are upgrade packages on offer for the model, this can prove to be a very beneficial method to getting further up the ladder in performance quality.

In some cases for not too much monies outlaid for a upgrade kit, a DAC can be produced that will hold its own against substantially more expensive models.  

I'll agree partially with Samureyex; Buy a new Gustard X16 for $500.00 anywhere.  I have been continuously amazed at how good this sounds, and if a new technology comes out you haven't sunk a big amount of $$. 

Definitely Denafrips Ares II 12th. Nothing I have owned comes close. I have had Schiit Bifrost 2, NuPrime DAC9-SE, Parasound ZDAC V2. The Denafrips has a sense of realism and a 3D like soundstage unlike no other DAC I have heard. Not taking anything at all away from the other DAC's it's just the Denafrips is that good. The Bifrost is also an excellent DAC, but for me, in my system the Denafrips (same price point) is a step above. You can get this unit brand new through Vinshine Audio for around $900 US. Their shipping is super fast (I received mine in less than 1 week) and their customer service is excellent!

Best of luck!