There is a slight "catch", but there's not much to it.
Depending on the third-party seller, there are different limitations on your service for 181 days. For some, you just have to maintain an active line and NOT switch your device to another line. For others you are unable to downgrade your plan or features until the end of the 181 day period. Be sure that you read Best Buy's terms to know what their policy is.
Basically, the worst that can happen is you're locked into the current data, text, and voice plan that you have for the next six months if you buy a phone from a third party. After that, there are no catches.
