Best CD player $500 or less used

A number of good choices have been mentioned but I would include the Cambridge Audio 640 C, both versions I and II. Obviously version I is going to save you a little more cash and version II is going to give you some upgrades in the D/A converter.

I'll add the Rotel RCD-1072 to the list. I had one and I am still sad that I sold it. VERY nice indeed!

The Denon DVD-2900 and DVD-5900 can both be had for around $200 and $500 respectively - both are highly recommended components, built like tanks, offer multi-format (CD, SACD, DVD, DVD-A, etc) compatibility, and are fairly common to find.

They both offer incredible performance for the price...just my $.02. Good luck!

They are a newer model, but look for a Pioneer
DV-58AV. An unsung hero on this site. Check out what
STEREOPHILE had to say about this vs the OPPO.
Great picture, excellent redbook and SACD.
Really good build quality for the price.
Good signal to noise ratio and great bass.
I have had many players up to $3500 and this one
really surprised me.

Easy. Rega Planet or Rotel 1072.