No one attempts to reply?
Ze, are you referring to online auctions, or any bidding games in general? For me, art auction houses like Sotheby's and Chrsitie's are old standbys, although reserve prices for their offerings are much too unreasonable. My favorite place in New York City is Swann's, a small company that specializes in 18th and 19th century art. If you have the knowledge, you will still find genuine works by second or third tier artists there. Another area to explore: tiny auctions or private estate sales.
The online art auctions are different beasts. One rule of thumb: you must know (1) what you're looking for (2) history of art/artists involved (3) be able to decipher fake signatures and/or the art itself. eBay, getting worse and worse daily, is still the better treasure trove. FB marketplace is a joke.
About what's hot in American and/or European paintings. You can look up various art price guides, either online or at local libraries; or at local art museum's archives/libraries.
About artists/art. That depends on what you're looking for. It's always good to buy art because you like it. Not because it will make good investment (which will never happen if you do not like it in the first place, no?