Have you ever been to a garage sale and touched one of the items for sale, only to find that your fingers come away covered in dust? It's disgusting and rather off-putting to discover that the host hasn't bothered to clean off the items they're hoping to make money from. As a host yourself, you need to avoid this mistake in order to put forth the best presentation possible to your clientele.
It's likely that a lot of the objects you're selling have sat around in a garage or basement for a few months or longer, and have gathered a fine coat of dust. Even if you do nothing else to prepare your items for selling, dust off everything. Use a rag and some dust-attracting cleaner and swipe it along all the edges of furniture, books, board game boxes, electronics, and toys (and whatever else you may have) that has the potential to gather dust. You may not be able to see the dust under artificial lighting, but once you take it outside, it's going to look disgusting.
Hose down bikes, lawnmowers, and other outdoor objects -- taking care, of course, not to get water on the electronic components. Anything with a motor may require a more detailed cleaning approach, with a damp cloth and some elbow grease, but bicycles, rakes, and other non-motorized outdoor tools can just be sprayed down with the garden hose. No matter your method, just get all the dirt and old plant matter off, and it will be clean enough to sell.
Before you even take them out of the kitchen, run dishes through the dishwasher. Nothing is more disgusting than finding food particles or lipstick stains on the flatware you're considering buying -- except maybe for finding fresh stains on the clothes you thought about taking home to your kids. You should also put shirts through the washer and dryer cycles, and, if you are a smoker or live with one, store them where the smell will not permeate the fabric again. Anything worn or used in such a way that it gets dirty needs to be cleaned before it goes out for sale. This will prevent the spread of sickness and also make your inventory less disgusting to browse.
The containers holding your items may have little bits of dirt and unknown crumbs at the bottom; take a baby wipe or damp cloth to the corners of your containers. Digging through dead beetles, spider legs, and halves of Cheerios makes hunting down treasures in the 25¢ a gross chore instead of a fun process. Do your customers a favor and get rid of those nasty surprises before they even have to sheepishly mention them to you.
One of the riskiest things to buy at a garage sale is baby toys -- just think about all the places those have been. Sterilize all baby toys (and all toys, really, if you can) in a diluted bleach solution and rinse them well. The same goes for any other infant-related equipment you plan to sell: bottles, cloth diapers, play equipment, or car seats. It all needs to be cleaned and disinfected if at all possible.
What kinds of nasty surprises have you discovered in your garage sale shopping?