Astronomy

Tl;Dr getting focus before you leave isn't SUPER important. It can be kinda tricky to figure out for newcomers, and you're better off using your time to assess the action and components of the scope.

Yeah, so, you're going to want to spend some time on YouTube U learning about how to focus a telescope. It could be trickier with a dobsonian/newtonian because you may need to collimate it (though the smaller the telescope is, the less important that becomes) to see clearly, and someone at a garage sale may or may not be willing to do it for you / trust you to do it. Generally speaking, though, if all the moving parts move like you expect them to and don't move like you don't expect them to, the lens or mirrors aren't obviously damaged or scratched, the eyepieces aren't obviously damaged or scratched (eyepieces are MUCH more sensitive to any kind of damage than mirrors or lenses in terms of user experience), then you've got a winner. It's hard to think of a situation where a telescope's parts would be in working order and good condition but somehow be broken in a way that prevents it from achieving focus.

For assessing movement: with a dobsonian in particular, but really any mount, you'll want it to be very easy (ALMOST but not quite frustratingly easy) to move the telescope so that you can track targets across the sky easily. The telescope, however, shouldn't move on its own without some force acting on it (i.e. touch, wind, etc). If it's moving under gravity, then either the balance is off (very possible with a dob, and usually easy to fix too), the friction is off (also an easy fix with a dob), or the mount could be bad if it uses some other kind of altitude-azimuth (left-right-up-down) mount that uses clamps and locking knobs and such. Also, make sure you try adjusting the focuser tube. They stick out and tend to get whacked, and if your focuser tube is busted, you're SOL. Just check to see that it moves when it should and not when it shouldn't, and that it goes all the way in and out without falling out; if it doesn't, check for some little thumbscrews on the focuser. Sometimes, when those are tightened down, they'll keep the focuser from moving, or let it move way too easy if they're too loose. If those thumbscrews aren't the problem, then the focuser is busted and you should give it a pass.

I also highly recommend checking the finder scope- that's the little buddy telescope that's attached to the telescope that's there to do business. They stick out a bit and have a habit of getting damaged. It's not a huge deal if it's broken, they can be easily replaced, but you're going to want to replace it before you head out or you're going to have a bad time. Lots of people love Telrad finders, I'm an absolute nut for RACIs; beware the little straight-through scopes, though, as they'll murder your neck when you have to look at something high up.

If you really want to try and look at something (not a bad idea, per se, just not the most effective use of your time. I'm assuming neither you nor the seller will immediately know how to achieve focus with the scope), try to pick something both big and very, very far off. If the moon's up, try looking at that. If not, try to pick the furthest, biggest thing you can see (big makes it easy, but if it's too close, you simply will not be able to get focus on it, period) and try to sight it in.