I don't buy many cards compared to the other folks around here, but in my (limited) experience, the scammers usually have red flags in their postings. Limited feedback is one. "I bought it at an estate auction/inherited it from my uncle/etc don't really know what it is" is another. "Listing as reprint/unknown" is another. It being a raw, high-dollar card is another.
Really, I think that the best advice is to not get greedy. Zillions of people trawl ebay every day, including everyone on this board. The deal of a lifetime is not going to sneak through. If there's anything iffy about a posting, just don't bid. It's only a baseball card, there will be another eventually.
As for spotting fakes, it's easier with the card in hand. For pre-WWII cards, a black light and magnification help a lot. (Incidentally, David Cycleback has a useful website:
http://www.cycleback.com/baseballcards/index.html ) On line, it helps to be familiar with the issue. I'm not a t206 guy, so I don't have much to say about the ones linked. But the Johnson and the Cobb have suspicious corners, they look clipped and then filed down to me. But those postings are so suspicious that I'd never bid on them even if the cards looked good.
Edited to add: the ones to be really careful with are the postings by people who don't really sell cards. If they usually sell, like, yard sale junk, and just happen to have a card, the posting itself won't have anything sketchy about it, but you also can't trust them to be able to spot fakes. I usually check seller's history, and deal with these guys carefully.