Need help with summing a variable number of cells

Thanks for looking in advance, guys. Just now made an account to specifically get help with this issue. I run a small business buying and selling used electronics (mostly video games) and I keep track of the numbers using a spreadsheet that I've customized over the years. Right now, I have no way to tell the profit from each deal except for summing everything in the deal, as shown here.
Screen Shot 2013-06-28 at 5.10.17 PM.jpg


After re-evaluating the purpose of the spreadsheet I realized the profit from each deal is crucial information that I think should be readily displayed and obvious.

Something similar to this: Screen Shot 2013-06-28 at 5.08.09 PM.jpg


However, because each deal contains a variable amount of items I'm having difficulty figuring out a way to automate this. I'm considering using the rudimentary copy and paste and just summing it myself with the mouse but I wanted to reach out and see if there was a better, more efficient way. Thanks in advance for the time and the help.

James