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

There are plenty of sellers who are sloppy about grading

My point exactly. If you can't trust anyone to grade correctly, then I would argue the grading system itself does not work. When you have a grading system that's mostly subjective, having so much granularity is always going to be troublesome.

I think a basic/non-granular grading system – New, Mint, Fair, Poor – would be more than sufficient and much less likely to contribute to the constant disagreements about condition between buyers and sellers.

@jeffdill2 

If you mean, for example, disagreeing about whether a disc is M- or VG+, I agree.

For some reason, the situations I run into are usually more blatant - disc's graded as M or like new that arrive with obvious scratching and or scuffing. There are still quite a few reputable sellers out there but if I buy from a seller that's new to me, I never know what to expect when it comes time to open the package. It does take some of the fun away from collecting.

Unfortunately, there is no grading system that can compel all sellers to be diligent/honest... not yet, anyway.

And we haven't talked about the rising prices !  I don't know if this affects vinyl but used cd prices have really gone up. 

CD's. If new music I buy from the band's website. If used: eBay. Specifically the declutter store. Best prices. Free and fast shipping. Huge collection.

I research all CD's on the DR Loudness Wars site to find the recordings with the best dynamic range and then find them on eBay.

We have a nice used vinyl store a few blocks from my house but I'm not into vinyl. The guy has lots of CDs. Would love to do business with him. Unfortunately they are not organized. No way I'm browsing shelf after shelf of CDs trying to read the vertical spine of a jewel case in hopes of possibly seeing something I'm interested in.

If I can find something on Amazon and the price is good, I'll generally go with that just because I know if there's a problem with it, returning it couldn't be easier. Otherwise places like Music Direct...