I've bought and sold. I've had only one mildly negative experience on both. Most have been good experiences. One, 1 of 3 lamps received was broken and all 3 special bulbs broken. The seller sent us a check for the value of the broken bulbs. The lampshade was unreplaceable, but... but I say! We already had one and only needed 3. Win/win.
Make sure you set up a PayPal account. You don't have to accept credit cards. If you don't accept credit cards your account is free. If you choose to accept credit cards you are charged 2.9% + 30cents per payment received.
Yes, make sure of who you buy from and/or who you sell too. If you are buying look very very closely at the pictures for defects. Ask the seller any questions before bidding.
Shipping can include a couple dollars for packaging costs. I tend to set a flat but reasonable rate as I put a lot of effort into preparing the shipment. Weigh the item with it's packaging before setting your shipping price online so you don't get burned. It's amazing how heavy things can get once packaged.
It's a lot of fun, and can be addictive!
Good luck.