I don't buy as much as I once did, but I still window shop the same places.
ebay
comiclink (auctions and exchange)
mycomicshop
highgradecomics
wwcomics
these boards, especially for some specific sellers
Myslabs isn't on my radar. I think I bought from them once, but the website's layout and filters make it too clumsy and time-consuming. IG and Facebook, I don't do, in part because I intentionally limit my activity on those sites, but also because it's just not a great buying venue imho. I know some dealers and sellers do really well on IG, and I even follow a few, but when I look at posts and such on IG, the layout bugs me. I spent a little bit of time buying on FB 2-3 years ago, but it was just a ****show of god knows who selling god knows what. And I don't care about live auctions. They are usually tedious personalities dragging you through reams of bad books to get to a few nice books; surely, some are well done, but it's just not for me.