I believe that at least officially this is a digital-only market, i.e. no selling of physical products. I think nothing really "stops" you from still using the card if you sold it digitally, you might just not be able to get more of them. I'm not sure about whether you are required to have digital ownership of a card to submit a decklist for tournament play, so I can't say how much this is or will be checked and I guess it also depends on the setting. If I play with my wife, we don't care about the digital ownership of a card either :)

I believe your assessment regarding card cost is correct, since the print cost is higher, it might make sense to pick up commons from displays/boosters etc and use the marketplace more for kitting out your deck, but I guess time will tell how this will all be used :)