Yes, but if you buy them from someone else, the Carrier (AT&T, Verizon) are still getting your money for the subsidy. This subsidy of $12.50-$18.75 a month for a phone is built into the price of the service no matter how you get your phone (difference in price of phones on contract vs off contract, dived by 24 months).
I would rather “buy” the phone for $199 or whatever, than buy the same one on the street for $500, and still have to pay the same amount to the Carrier.
If an unlocked phone is worth $15 a month to you, then go for it. For me it’s not.