Depends, tears and scuffs could just indicate that it's been toured with and banged around but don't necessarily mean that anything internal is damaged. I ask lots of questions, including how some of the scuffs got there. I check to make sure that knobs, jacks, etc. are all tight. I check to ensure the tubes aren't red plating. I always play for at least 30 minutes at various volume levels (including really loud) to get a feel for how it handles power and if there's any major issues, those will likely show up within those 30 minutes.
I don't do this to private sellers, but if I'm in a store like GC and an annoying sales clerk is pushing me to buy, I'll also adjust things to sound worse than the amp actually is just to try and haggle a lower price on something that doesn't sound so great
If it passes those tests and I like the amp, I buy it. If I don't like it, am unsure about the durability or still have questions, I point those things out to the seller (haggle a little depending on how major the issues are) and then sleep on it. Sometimes I'll ask if I can come back another day to play it again, or if he wouldn't mind me playing it a little longer to be sure. The point is, I keep testing until I'm satisfied with an amp and then I either buy it or walk away.
It's annoying as a seller to have someone walk away so, when that's me, I always try to give my reasons for why I'm not just handing them the cash. I've tried selling enough things myself and had people leave me hanging and it's not fun, but at the end of the day, when I'm the buyer and it's my money, I want to be sure.