Yes, you do have to pay for the shipping, so I usually only ship large orders and I ship them the cheapest way I can. As for how much you make, it depends on what you're selling to them. Things that are currently popular bring higher prices than old movies or albums that they either can't sell or already have a ton of. They list the prices that you can get on the website when you go to the "sell" area -- if you have something not listed, you can email them for a price quote.
Like I said, I've generally found their prices to be higher than those I can get through a yard sale or another trade in shop locally. But maybe it's just what I'm selling and I do ship a lot of stuff at one time to offset the shipping costs. Also, I usually just get store credit from them so I can buy something else later. They pay the same price whether you choose cash or store credit, but since I'm usually buying something all the time, it's easier to get credit. I'm a music and movie freak and buying and selling through secondspin has saved me alot of $$$ over time. It lets me feed the addiction without paying retail. I love them, but I guess it depends on your persoal situation as to whether or not it'd be worth it. HTH.