What is your go-to online place to buy physical medium of music? And why?

What genre of music are you seeking?

I buy most of my CDs from Amazon when they have the title I want. They typically have great prices and with Amazon Prime I get free 2 day shipping.

I've bought most of my used LP's on eBay because they generally have a good selection available and I typically find a decent price. I'm not going to pay the high price of 180-200G new/re-issue LP's for most selections, so high quality used items are what I typically buy. I've also bought my fair share of used and new CD's on eBay.

For vinyl no online purchases under any circumstances unless purchasing SEALED or purchasing directly from band. Condition of vinyl that joins my personal collection must be like new and not any less.
CDs I used to get from Columbia House or BMG depending on who's got best deal. It was long time ago. Now these clubs don't function. I was always making my membership new -- this way you get the best deal.
Once membership is not new, I'd cancel it and wait for another new membership invite keeping price per CD $2...3. When CD started being out of print, the time to sell would come and make some money :-)
For rare copies I'd shop in discogs(look for reputable dealer with nice feedback). Prior to discogs there was gemm.com and cdnow.com

I use Elusive Disc almost exclusively, with Music Direct and Soundstage getting maybe 10% of my orders, this is for vinyl and I use Amazon for CDs. Haven't been in a record store in 20 years. I also haven't been in an audio salon in 25 years and just completed my to die for reference retirement system.

Amazon  has been an excellent source for me regarding new and used CDs. Broad selection, good pricing and reliable shipping. I have no complaints with them.
Charles,