Best CD player $500 or less used

I'd look to a used Rega Planet 2000 or Arcam CD73. Good sound for dollar and good dependability. Should be $400-$450. Another convenient option may be to get a USB DAC and run from your PC if you keep a music library on it.

For that money, I'd try to find an Audio Refinement Complete. Depending on personal taste, you may also want to have a look (better a listen, if at all possible) at the Opera Consonance cd120. Less musical flow i.m.o., better detail, others will hold.

I have really enjoyed my Music Hall CD25.2. Has a nice slightly warm rich sound and can be found used for around $300, new for $440.

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.