Garage sales sell just about anything you can imagine, but that doesn't mean you'd want to buy everything you can imagine. Approach garage sales with the same mentality as you would a second-hand store; if you wouldn't purchase it at your local Salvation Army shop, you probably shouldn't buy it from a garage sale.
Objects that are rusting in a way that cannot be polished off or removed should be left alone. Rust can cause an item to become unstable -- electrically, structurally, or otherwise -- and unless you're absolutely positive you can get past the rust damage, you don't want to spend your money on something that for all intents and purposes should be in the garbage bin.
Never, ever purchase open food. Hopefully, the host has enough sense not to put any out at their sale, but occasionally something will slip under their radar (or, you'll run into a careless host -- let's hope not!) and get mixed in with other products. It's safe to assume that any food at all for sale at a garage sale isn't good enough for the host to keep, so you have every right to be skeptical about its origin and current state. Yes, you may very well get food poisoning from that can of beans. Yes, you were warned. Opened food is even worse, because it means any factory seals that prevented the entrance of bacteria into the food container have been broken.
Okay, this one should be common sense: underwear and socks that have clearly seen use are just disgusting. You have to be pretty desperate to want to touch that, much less wear it. Think of this: many garage sale hosts don't wash the clothes they're putting out for sale. Yeah. Now are you so eager to buy up those 10¢ socks and 25¢ underwear? Thought not.
Hygiene and beauty items can get pretty disgusting if used and then left at the bottom of a bathroom cupboard for years. Christina Heiska of The F.U.N. Place once found a crusty eyelash curler at a yard sale. EWW. Even a hair straightener can get pretty sketchy after years of burned hair residue building up on the irons. If you do purchase hygiene or beauty items (and please don't buy any opened toothbrushes...!), make sure to sterilize and clean them thoroughly before your first use.
Anything you plan to use for survival, such as rope, waterproof blankets, or other wilderness gear vitals, are best purchased brand new at an outdoor equipment store, or at least at your local department store. While they may be perfectly adequate for the tasks, you don't want to bank your comfort and maybe even your life on something bought second-hand unless you're sure it won't fail you.
Don't buy any items you know are recalled or potentially dangerous. That's just plain foolish. Some common items are infant toys and supplies, children's clothing with drawstrings, some propane grills, and many toys with moving parts. If you aren't sure, use a portable wireless device or ask someone if they can look it up for you (a call home to someone near the internet will suffice).
In a list of things you shouldn't do, here's something you should do: visit GarageSaleCow.com!