Best CD player $500 or less used

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.

California Audio Labs CL-15
Musical Fidelity Electra

A Planet 2000 just sold for $350 in the classifieds. It's a nice player that usually goes for $400+, but I didn't find it any better than a Denon 2910 which goes for $200-250.

If I were in your shoes, I'd be looking for a Jolida JD100. Its tube output stage differentiates it from other low cost players. To my ears, it's much smoother and more natural sounding than either the Planet or the Denon. And the build quality is very good.