Can a used Mercedes 2014 B200 fob be reprogrammed

The linked answer in the comments is incorrect. You can in fact unlock them, However it does not make sense to do it in most cases and rarely is it done for newer keys. Only specialists will do Mercedes keys to begin with and usually we will only want to use our own stock that we know the quality of. You will not save money by bringing a key to use.

You can not do it yourself unless you own the proper tools which are specific for the task and fairly expensive.