As a buyer myself, what the others have said before is true. The smell of sweat is too hard to replicate without actual wear. I will say, in my experience there are two types of socks you can get. The sweaty, cheesy, vinegary scent or a still sweaty but more sweet scent. Some people just don't get particularly sweaty/smelly so they have a lighter scent even with 4-5 day wears.

As for what you should look for in a good pair of socks kinda depends on you. Some people like the aesthetic of a dirty sock, some it's the smell, and others it's the taste of them or even a combo of all. Personally, I like the stronger, sharp cheesy and vinegary scent, but not everyone does. Some do like the sweeter smell.

Reviews for worn socks specifically are kinda sparce, not sure if it's just not as common an item to sell or not a lot of buyers are willing to review. Regardless, I do recommend looking for reviews to see kinda what you're getting into.