Used DACs for sale - help me choose

Get a Lampizator 

Anything by MHDT very musical

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 $$.