If you HAD to purchase a used (over 4 years old) DAC, what would it be?

Chord DAVE.  I have my Lampizator otherwise engaged right now so I'm listening to my DAVE and I am glad I didn't get around to selling it.  It is a great buy at the current price.

I've always wanted an older Levinson DAC like the 360S, that's probably where I'd be shopping.  :)

How is one supposed to make a recommendation when nothing has been shared about what the DAC will be playing with or your listening preferences?

There are a few terrific sounding DACs that are pretty much unchanged over MUCH longer than four years.  I would take a twenty year old Audio Note DAC 5 over almost anything out there right now.