When to Sell Your Stuff
Once you've figured out what you can sell and how to sell it, the next question you need to ask is when. This is particularly important for tech gear that you need to stay current on; with equipment like work PCs, high-end photo and video cameras, and smartphones, selling before the big announcement of the next-generation gear could mean a difference of as much as 10 to 20 percent of your selling price--and in just a day or two.
Work Your Warranties
If you know in advance that you're going to resell a piece of tech gear, you probably want to buy an extended warranty on it. Extended warranties are great for driving up the resale price of a used PC--after all, a PC with a manufacturer's warranty on it is worth more than that same PC without.
Buyers are also more likely to buy a used PC (or camera, or other device) with a manufacturer's warranty because buying a used PC online is like buying a used car online--you don't even get a test drive, much less a checkup with a mechanic. If you buy a lemon and the seller vanishes, you might still be able to get a usable PC out of it.
Even if your warranty is nearing the end, however, you can still get it to add a few bucks to your sale price. I just prepped a first-generation MacBook for resale, and the first thing I did was take it in to the Apple Store in the last few days of warranty coverage because it wasn't recognizing ethernet connections.
While the techs had it, they found a whole bunch of other problems that I hadn't, including a monitor inverter issue (which caused the display backlighting to dim), a hard drive close to failure, and some problems with the logic board--all of which got replaced (and the hard drive upgraded to 160GB since they didn't have any 60GB drives left), complete with a clean Snow Leopard install and a few cosmetic parts replaced.
Not only are the guts of the MacBook near brand-new and the exterior nice and shiny by the time I resold it, but the receipt gave me proof of recent warranty coverage (to show to the prospective buyer), and if the MacBook needed any more work related to those issues, they'd be covered for 90 days even though the warranty expired. That practically paid for the $300 cost of the warranty when I bought the laptop.
Timing Is Everything: Trade Shows and Product Cycles
It used to be that you could count on all the new tech gear for the year to show up at big trade shows, such as CES, MacWorld, and E3. As a result, everyone dumped their stuff right before the show, while it was still fairly current, and bought the new stuff as soon as they could.
Lately, it's not quite so predictable as it used to be, given that larger companies seem to just announce new stuff whenever they feel like it. But it's still certainly worth following product cycles for products you're tied to, especially if it's a certain brand or product line that you're depending on.
If a new BlackBerry aimed at the international traveler market just came out, you can bet that another one probably isn't going to come out for six months at the earliest (probably closer to a year). Also, keep in mind that the end of November through the beginning of January (from Black Friday to Christmas and CES) tends to be full of heavy discounts and new product announcements, meaning you probably want to sell your stuff well before that season to avoid competing with slashed retail prices.
Just Sell It Already
Warranties can also give you a good idea of the manufacturer's intended lifespan for your gadget: Once something is so old it can't be covered by a manufacturer's extended warranty, it's most likely old enough that any further depreciation will be a matter of years rather than months or weeks, so either sell it for a pittance or use it until it stops working.
For example, if you have a perfectly good point-and-shoot camera that's three years old (an Olympus FE-280, for example), but you're itching for one with better video capture, higher resolutions, and better in-camera features, you might decide that the $23 you could get for it from Gazelle isn't worth it. If it won't tide you over until the camera you want is more affordable, find a creative use for it--or give it to someone who would make could use of it.