I think the classic "shareware demo" model is a good one, have some free stuff to introduce the user and make sure they like the game, then give them a wall and tell them to pay if they want more. It's fair to them and fair to you. It doesn't violate their privacy, it makes sure your customers actually want to be your customers (reducing chargebacks because the game has an issue for someone) and you get paid without polluting your art.