Best CD player $500 or less used

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.

Cambridge Audio 640c v2

Onkyo DX-7555. You can buy these new for 400 bucks these days.

Sony PS-1 !!!!!

I think the Rotel RCD-1072 is very hard to beat for the used price.