If it's full auto, there are many Federal rules and Regs, but the short version is
if there is no blue ATF tax card, walk away. The weapon can only be transferred through a certified Class III dealer and you will have to apply to BATFE for permission.
Full auto is legal in Tennessee, but only with BATFE approval. Please understand, I am a nurse, NOT A LAWYER, so go to the BATFE web site to look at the rules.
If it is semi auto, then it has to fire from a closed bolt and has had a new receiver installed ; no further registration is necessary.
Side note: a good friend bought a pistol from "a guy at work" that he trusted. He eventually swapped it to me and I tried to trade it for a shotgun I wanted a local gun store. Turns out it was stolen. Friendship doesn't guarantee that you won't get a stolen weapon.