You earn money after each game. When win, get 40$, lose - 10$ and draw 25$. You can also collect cash selling cards (you put cards from own collection for sale and maybe somebody will buy it, lowest price is always first). There is also option to ask players for $. All people online are very friendly so it shouldnt be a problem. When you want to play sealed, also opponent can help you and give some money for products.
In sealed mode you can only use cards just bought (its good to use option refresh before making deck, sometimes you dont see some cards received from boosters/starters). But you have access to all sites(here is difference, in RL you use sites found in products).
For sealed game limits of cards are 25/25 and 10 creatures. And you can call after collecting 18MPs (dont need to exhaust deck) or any result when you finish you deck.