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

 Acoustic sounds has incredible customer service. If you buy vinyl from them, and the record is not up to your expectations, they will gladly replace it. As they  also own quality record pressings, they understand that not every record come off the press in perfect condition, yet they still get sold!  My experience buying vinyl from quality record pressings plants is different. They have very high standards Ann most of the records come out perfectly.  This  variance in pressing quality and return policies is relatively new information for me. I never really knew that I could exchange and get something that was actually better. I didn't realize that pressing yielded a significant quantity of duds.   I called the people at acoustic sounds recently to ask about their recommendation for a cartridge stylus cleaning fluid. The gentleman I spoke to spent a half an hour with me on the phone sharing tons of great information. I've purchased plenty of media from acoustic sounds, but now I will plan to do all if not the vast majority of my purchasing from them!

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