whag is there something I should look out for someone who replaced a transmission? And why would someone have to replace a transmission?

Transmissions go out for varies reason,s,driving hard mainly,and from not being cooled right from bad radiator,or leaking tranny fluid,test drive it for a long run,then check the fluid for burnt smell and to be red not brown,then look under it for leakage,if fluid is brown and not red,walk away and find you another ride. Good luck.