It sounds to me like you were sold an unestablished baby, and that it is starving to death.
At this point, it's probably too weak to eat on it's own, so you should contact a vet to try tube feeding to get some nutrition into it or it will die.
I do NOT recommend trying to tube feed it yourself, it is not something the inexperienced should attempt.
If the vet successfully tube feeds the baby, you can try to offer it live food in a few days. You must act quickly, or the baby will not make it.
Once the baby has eaten a few meals, you can switch back to F/T. But be aware if you do not heat the F/T mouse sufficiently the snake will not pay any attention to it.
The sad fact is, a lot of people sell babies that have never fed or have only eaten once and getting a baby to eat is not always an easy thing to do. It's a problem I myself have had once, before I knew better and to only buy from reputable people who can provide a feed record for a baby I am interested in buying.
Gale