As a rule, start by taking 10% to 30% off the original price. This is a great starting point, especially when pricing pricier items such as TV's or stereo equipment. Remember that even if the item is brand new, it loses its value because it is out of the box and is not being purchased directly from a retail store. When deciding how much to depreciate by, consider the benefit that the consumer would have from getting it at the store; for example, a warranty.
Once you have discounted the item, it's time to depreciate it. Consider the age, condition and the life expectancy of the item. Also take into account the demand for the item. It may be a great buy, but who is going to buy it. A good example would be a black and white television. Even if it is still in its original packaging, you are not likely to get much for it as it is an out-dated technology. This is very important to consider when pricing clothing as well.
If you are still confused about the price, look on-line at similar items. Many of the items found at garage sales can be found on sites like ebay.com or amazon.com. In general, these prices are going to be a little higher than what you can actually fetch at a garage sale. Also, check sites like garagesalesource.com to see what other people are selling similar items for.
If you have higher priced items, price them slightly higher and be willing to barter. You are more likely to sell for an acceptable price this way.
Often we put things out at the sale and then regret letting it go for so cheap. If you have sentimental items, make sure you are OK with selling them and price them in a way that you will be OK with letting them go so as to avoid regret.