Best CD player $500 or less used

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.

A used Eastern Electric MiniMax is close to that price and sounds great plus you can tailor the sound with tube rolling my suggestion is some Amperex tubes. Also it's a must to put it on cones or some other type of isolation it makes a HUGE difference.