Ya see, Philadelphia is the main mint so they didn't figure they needed to put a mint mark on the coins they produced. The others are kind of branch mints so for one reason or another someone decided that there needed to be some way to identify the coins from each mint. As the other mints started producing coins they put a letter on them so everyone could tell where that particular coin came from. I guess the
RedBook classifies things the same way.
The folks on
ebay can say whatever they want, they can advertise that a coin is a 1947 p even though it has no mintmark mostly so that people know which mint it came from I would imagine. They probably didn't mean that it has a mintmark if it doesn't, they're just making it easier for buyers to know what they are getting.
Unless you're looking to get an idea of what coins are selling for currently
ebay isn't any kind of information source, it's just a big auction.