Prepare Your Products
Before you make the sale, you'll want your used tech looking its best. Dings and scratches can take a hefty chunk out of the resale price, and eBay buyers might have grounds to initiate a PayPal dispute if they discover damage that wasn't reported in the auction listing.
You'll also want to protect yourself--if you're selling something with onboard memory or a hard drive, you'll want to make sure that your data is cleanly wiped before handing it off.
If you're selling your laptop, you'll want to take care cleaning it up--read "Caring for Your Laptop" for a few tips on cleaning the keyboard and display. If you're under warranty, you might be able to get some chipped or cracked bezels replaced. Also, check out "How to Sell Your Old PC" for more prep tips.
Desktops are easier to clean than laptops because you have more access to the inside. Start by opening it up and grabbing some canned air from your local electronics store to blast out that dust.
If you don't have canned air handy, you can use a low-power handheld vacuum--preferably using the narrow tapered nozzle--to suck up what you can.
Once you've tidied up a little bit, use some twist-ties to keep the internal wiring as orderly as possible, and shoot a few pics so potential buyers know you know your stuff. All the tips in "Preserve Your Mac's Resale Value" apply to PCs, too.
Don't bother upgrading any of the hardware before selling it, because you'd be lucky to make the cost back in the resale price, but definitely install a clean version of the OS (the newest one possible would be best).
Also, if you don't want to (or can't) include your software licenses for common apps like Microsoft Office or Photoshop Elements, consider downloading a few open-source applications like OpenOffice and GIMP instead.
Keep in mind that if you're a regular indoor cigarette smoker, your gadgets may have suffered. Smoke can stain plastic cases, get sucked into PC fans and accumulate inside, and leave a long-lasting smell on your electronics that don't bode well for resale purposes, and can actually be a health hazard to people opening up the case.
Cameras have a few specific cleaning tips, especially if you're selling a DSLR model (point-and-shoots typically don't give you as much access to the camera's guts). You'll want to look for fungus growing inside the camera, in the shutter, and on the mirror.
If the camera has been sitting around for a while with batteries inside, the battery can sometimes corrode and leak, so clean this up carefully and replace with fresh batteries.
After that, make sure the memory card compartment is clean and none of the pins are bent. If the lens filter is scratched, take it off or replace it with a new one--they tend to be pretty cheap. Those of you who are completely confident in your camera repair skills could clean the image sensor yourself--it can add to the potential resale value and save you the $100 or so that a professional store would charge.
However, you can also risk causing permanent damage, so don't do it unless you know what you're getting into.
Printers typically have their own self-cleaning modes, so there is little you can do outside of running it through those a few times. Make sure you have the USB/ethernet cables (if necessary) and power cables, because they're a pain to replace.
Also, if you're selling online, include a scan or photo of the printer's test print page so prospective buyers can see that it's in good shape.
For all PCs, hard drives, and other devices with memory (cameras and memory cards, portable media players, smartphones) you'll want to do a clean wipe of all your data. If possible, use DBAN to completely clean out your storage so no one can undelete your data and start snooping through your old files.
Smartphones, media players, and cameras should have a "Factory Wipe" or "Restore Factory Settings" option that reverts your device back to a factory-clean condition.
Also, don't forget to transfer any purchased downloads to a backup device. While Steam purchases can be redownloaded, music and movies from the iTunes Music Store generally can't. Check with all your online media stores before you clean up your data because it might not be as simple as downloading it all back.
Finally, if your tech is still under warranty, make sure that you've figured out how to transfer that warranty to its new owner. Depending on the policy, it might take just a phone call--or it might require that you and the buyer both show up at a store, which would make cross-country eBay sales a little bit tricky.
No matter what you're selling, you'll get a better price for it if you have the original box, manuals, included software, accessories, and cables.
If you're missing some of them, it's probably not worth your while to try and track them down; even for necessary items like batteries and power cords, you'd be lucky to make back what it cost for you to buy another one in the final auction price.