Pros:
You get the whole setup, and a snake that should be comfortable in its environment.
You get to talk to the previous owner about what the care/personality was when they had it.
You get a cool snake for a cheaper price than the pet stores.
Cons:
You may get something you were not prepared for, aka sick snake, malnurished snake, extra care snake, stressed out snake, etc...
You may be given something completely incorrect as a setup, and have to start out again buying all new supplies, etc..
There is more, but it is lunch time, and I'm about to dash. You get the idea...