Depends...if you are talking about the liquid goo in the bottom of the smoker, that could be a lot of grease and fat - which I will WIPE out with wadded up newspapers...
However, in general - the black stuff sticking to the inside of the smoker is the seasoning - so you can leave it.
That said - it also depends on the previous owner. If the person didn't know what they were doing, that black stuff could have a lot of creosote in it, or it could be seasoning from bad wood.
Therefore, when I bought a used smoker last spring, I cleaned it out and re-seasoned it - because you never know what the last guy did to it!
Have fun!