There are many factors that affect prices: season, region, condition, age, current retail price, and competition.
Selling items in the off-season makes them a little harder to move; most folks just arent looking for Christmas decorations in May, so keep the seasonality of items in mind when pricing. Ditto for gardening tools. Not much of a market for them in September (unless youre selling a leaf blower!), but they are in high demand in spring. Sweaters and jackets will bring more in an early autumn sale.
The part of the country youre selling in plays a big part of what you can hope to get paid for your goods. Garage sales tend to be more prevalent in the suburbs and rural areas, whereas flea markets are more common in urban areas. The general cost of living in your area will also have an effect on prices. If you are in an area hard-hit by the recession, folks just dont have as much money to spend, so your prices will have to be low enough to move your inventory.
Is the item like new? Slightly used? Does it come with its original packaging and/or instructions? Are all the parts included? Or has it obviously been collecting dust in your attic for several years? Dirty stuff is harder to sell, so take the time to clean stuff up.
Some things are timeless, and some things are even more valuable because theyre older, but just because something is old, doesnt make it a collectible or antique. Anything you plan on marketing this way will need to be researched. Ebay is probably your best source for current prices (be sure to check the COMPLETED auctions for actual price data). Clothing usually continues to go down in price the older it gets. Other items that get dated particularly fast are home dec items. Remember those acrylic bunches of grapes everyone had in the 60s? Not a big market for them now. Ditto for avocado green Tupperware, etc.
Current Retail Price:
It doesnt really matter what you paid for an item several years ago; todays prices are all that matter. So, that computer that you paid $1200 for 5 years ago, is likely not even going to get you 10% of its original price. This is particular true with technology items: the costs have dropped so dramatically in the past several years that theres really no good secondary market. That goes for kitchen appliances, DVD players, etc. (In 1986 a new full-size microwave would set you back over $300, now you can get one for less than half that price.)
Where else can shoppers find the items you are selling used? Is everyone else in the neighborhood having a sale? Are there many second-hand shops in your area? How unique is your item? Is it something people buy and use forever, or is everyone else also selling the same as seen on TV widget.
Oh, and finally, how bad do you want to be rid of the item? (I actually had a guy offer to pay me to take his exercise bike away last year!).
I recommend heading to a few second-hand stores in your area to get an idea of prices, then charge about 70% of what they do. If you have things that you feel may be valuable, check eBay for prices. If you dont sell these items at your sale, you can always sell them online.
Some of these prices seem shockingly low, so you may want to consider whether donating the items to charity will net you more money as a tax deductible donation. Many popular income tax software packages, such as Turbo Tax, include features to automatically calculate the value of charitable donations using IRS-approved values. At the very least, make sure you take inventory (a digital photo for support is a good idea) of items before donating them to charity.