You're at a garage sale on Saturday morning, and you spot a 35mm SLR on one of the tables. The owner is asking $10 for it. You're wondering if you should take the chance. Here's what I suggest you look for, and ask about, to help you make a decision.
First, ask the owner if the camera works. I know this sounds pretty obvious, but I learn a lot with this question. Normally the answer is, "Last time I tried it, it worked perfectly. But the batteries are dead, I can't test it now."
OK, fair enough. Next, remove the lens and look inside the body. If the mirror is in the down position, that's a good sign. If the mirror is in the up position, especially if it's a Pentax, then move along. It's not for you. Mirrors stuck in the up position usually have a broken gear that's very difficult to fix.
If the mirror is down, take a look at its surface and the surface of the fresnel viewfinder screen. There will most likely be dust on them. That's OK. You can blow that off. But if there are signs that the owner tried to clean either surface, then that's a deal breaker. Fact of the matter is, no one knows how to clean a SLR mirror, and it's probably ruined. You can double-check this by looking through the viewfinder. You will probably see smudges on the screen. Put it down and move alone.
If everything checks out so far, then inspect the mirror bumper. This is the strip of foam that the mirror bounces off when you take a picture. If it is gooky, as old foam becomes sometimes, that's OK. Just know that you're going to have to scrape it off and replace with a fresh strip of foam. For me, this isn't a deal breaker because I do it all the time. I'll leave it up to your discretion. (Any camera repair service can take care of this too.)
Next, inspect the outside of the camera. Is the paint in decent shape? Do the buttons and dials work properly? Can you advance the film lever? (If the shutter is cocked and batteries are dead, you may not. So this is a judgement call.) What shape is the leatherette in? If it looks terrible, that's not a problem. Just add $15 on to the price of the camera. That's how much a replacement cover will cost you. You can do that repair yourself.
Now check the battery compartment. Is it clean and in good shape? Yes. Fantastic. Is it a battery you can purchase in a store, such as an LR44 button or CR2 Lithium. Yes. You're almost to a purchase.
Finally, open the back and inspect the film compartment. Is it clean? Does the shutter curtain appear to be in good shape? If yes, these are both good signs. Now check the light seals. There should be a strip of foam at one end or both that keeps stray light from entering the compartment. If it's in good shape, yay! If not, know that you will probably have to replace it. I do that at the same time, using the same material, as the mirror bumper.
This casual inspection should only take a minute or two. But if the object of your desire passes these steps, then I say the camera is worth the risk. Best case, you get a new shooter to experiment with. Worse case, you have a souvenir from your morning of bargain hunting... and a good story about how you almost scored a great deal.