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

I would find a used Ayre QB-9, or QB-9 DSD, and then send it to Ayre to be updated to the QB-9 Twenty. I think it sounds wonderful in that new form, and both buying the used DAC, and then getting the upgrade would likely be under $3,000 total.

Hard to go wrong with a Denafrips Pontus if it’s within your budget. 

I recently picked up a (10?) year old Mytek dac on the cheap and I'm quite happy with it.

Depending on budget, Value ,dependability. Sonics.

I'm pleased enough with my Denafrips Venus 15 that I'd go for a Denafrips Terminator 12 or Terminator Plus, whichever of those were made 4 yrs. ago.