There are two big rules:
1.) There is not one single best vendor
2.) A vendor is good, until he is not anymore
Let me explain.
In the grey market you are concerned with multiple things. First, you do not want to get scammed (they are lurking everywhere, read the anti-scam tips).
Second you want to get qualitative products for a reasonable price, that finally reaches your door.
Ok, I get it. You want to just know where to buy from.
But you are not on amazon prime. The grey market has its own rules and I try to tell you how to navigate that space.