Depends on what you're selling. If you're reselling stuff it's really hard to make any money unless you have a huge margin (you bought something for $1 and can sell it for $50). Most of that money will go into all the fees, and shipping. Also it's really hard to be an independent seller on eBay because there's so many people out there that can offer free shipping, and most people can't do that and still make money.
If you're just trying to get rid of crap around the house, keep shipping in mind. Selling a snowboard online can be expensive to ship, and finding a box for that is a pain.
If you're selling a lot of stuff, it might be worthwhile buying some shipping supplies from someplace like esupplystore or uline.
Printing shipping labels on your own is usually cheaper than going to the Post office or UPS store or whatever. You will probably need access to a weighing machine though...
Also, make sure you have a good feedback record. If you have a feedback of 0, or anything lower than 100%, people will be hesitant to buy anything from you.