If you were a garage sale host and saw someone step out of their Porsche dressed in a long fur coat, with their little dog under their arm, would you be inclined to give them any discounts or leeway in bartering? Probably not -- they can clearly afford to pay you the full $3 on that silverware set. Dressing to yard sale shop is very important for both your first impression to the host and your own comfort; you should choose your outfit carefully.
Take a step or two outside, and check the weather forecast, before you decide on a shirt to wear. You may want to start by layering -- wear a t-shirt over a tank top, for instance -- and then you can remove layers as the day warms up. For appearance's sake, you should probably wear something casual, like a plain tee or one of your old favorites. Anything that you can't wear to the office is acceptable, and probably the most comfortable anyway. (Remember: you're not trying to deceive the hosts of the sales you visit; you're simply avoiding pitfalls that will give them the wrong impression and make them treat you differently or even rudely.)
The same weather-checking tip goes for pants. Jeans are a classic choice, but make sure you pick a pair that are well-worn and broken in -- otherwise, you could end up chafing by the end of the day! You want something that is easy to move in, but sweats are just a bit too casual. Shorts are also a good choice, as are capris.
Do not wear flip-slops. This is for your own safety and comfort. Flip-flops require you to squeeze your toes to hold them on, and provide no support whatsoever for your arches. A comfortable pair of tennis shoes is your best bet for garage saling, especially if you plan on walking instead of driving your routes. Fancier shoes are not recommended either, as they make you seem "too good" for garage sales (and are rarely as comfortable as sneakers).
Women can wear a dress or a skirt, if it's casual, but anything fancy is out. The same goes for suits; it just makes you seem as if you're deigning to stop by a garage sale, and you may be met with hostility. If you've got a casual watch, you can wear it if you need it, although in this age of cell phones it might be more efficient to carry yours around to check the time. Caps are useful to keep out the sun, along with sunglasses, but designer items should be avoided.
Whatever you wear, make sure you can accurately measure to it. Since clothes at garage sales are generally not available to try on, you'll need something to compare it to that you know fits you. If you wear a too-big or a too-small shirt, pair of pants, or other article of clothing (like a jacket), you won't get an accurate idea of how the potential purchase might fit you. If you are dead-set on wearing something unusual on your expeditions, take another shirt or pair of pants along and keep it in your car.
When it comes to dressing your kids to take with you, keep these tips in mind. Help them choose comfortable, casual clothing that will keep them warm or cool enough and allow plenty of movement.
Check out GarageSaleCow for some other great resources. You can design your own sign, read up on more tips, or list your garage sale for free!