In the U.S., many times there are buy one, get one offers on popular phones. Verizon has a promotion for the S7 for buy one, get one free. So someone could do that, pay off whatever payment plan they need, then sell the "free" phone for a profit. I've also been able to upgrade with a promo code that let me pay $8 for a Droid Mini. I flipped it for $50, and kept my G4 active on the line.