To some extent, it depends on where you live. I'm in a small town, and the nearest areas served by Kijiji and Craigslist are either 50 or 100 kms away. The 50 km one is a small city, so there tends not to be a lot of interest there.
Nonetheless, I would try Kijiji and Craigslist first, just for a couple of days, before going to eBay. In my experience, if it doesn't sell in the first couple of days on those local means, it probably won't sell at your current price. I find people buying on those lists usually are quite price sensitive, looking for real bargains and not really knowing the worth of the item, while sellers tend to overestimate their prices by a lot. Last week, I saw someone on Kijiji offering a Mac Pro for $2700 when the same model could be bought refurbished from Apple for $2200. Be prepared to do some negotiating. You will need to be ready for the fact that a buyer will probably want to see the computer working, so you may need to be willing to meet with them in your home or office.
eBay, on the other hand, gives you a way to figure out what you would probably get by looking at completed auctions, so you have an idea in advance what things will sell for, and you get a wider audience. The starting price can be tricky. Both eBay and Paypal take fairly substantial cuts of your profits, and you do have the headache of packing and shipping, a real problem if you don't have the original box. BTW, the advice about photos and saving shipping receipts is excellent.
You might want to look for interest groups that offer buying and selling, like ehmac in Canada.
Good luck!
