Wow! Broad question. The reason I say broad is that there are a lot of ways to approach auctions. If you bid in an auction that is hyper publicized, super well attended, and the buyers are flush with cash..........and you win, I have little doubt that you will be paying absolute top dollar. And say 5 years from now you sell same coin, in a recession, at a smaller auction, not as broadly successful, you will take a beating. A big one.
Another big question is what type of material interests you? Auctions that have floor bidding really need to have some kind of minimum per lot to make the auction economically feasible. Very interesting Roman Provincial coins might not make that minimum, and yet that might be what interests you.
I also posit that different auction houses specialize in different material. You almost have to specify what types of ancient coins interest you, at what kind of price levels (2 figure, 3 figure, 4 figure, 5 figure) to get the right pointers.
Oh, don't forget the European auction houses. Great great stuff. A little more complexity because of payments but also a good vector.
Paul