I have traded on Tradeit.gg and DMarket, but I'll usually do basic shopping on Skinport. (Only reason is because Skinport doesn't ask for ID verification to make purchases; I would buy on DMarket for better prices but it's too much legwork to verify, whereas trading for same-value is just an exchange of items not requiring deposit, thus not requiring your identity.)
I heard buff is where most people go
Buff163 is alright but I like Bitskins P2P because the pricing is clearer and transactions are easier. I also use Skinport for quicksell because its pretty active with a good market
trade with friends, trade discords, etc.
you can't trade on skinport
buff, and possibly skinport (but only if you get a -25% discount) in terms of ther market prices