101cd and wow are mainly cheap imports from god knows where, i used to buy from them, but so many had different covers and tracklisting to either uk or us versions.
However if you are a collector and want rare or oop stuff, there are only a few really good sites out there: CDNOW.com - Gemm.com - MUSICSTACK.com - SECONDSPIN.com and someone has already said DISCOGS.com
If it is just mainstream pop etc then the ones you listed always seem to be the best.