You pay $20 to buy a movie
The value of that movie drops by $1 per day (maybe $2 if its HD)
You can sell it back at any point if it still has value and you are credited to your vidangel account.
So basically you can rent a movie for $1 if you sell it back within a day. If you ever quit using vidangel without ever buying a movie and keeping it, you will leave up to $19 of credit on the table.