I use a excel spreadsheet that I have built.
It is full of formulas.
I enter each sale I have with the date, account, item, price, shipping, cost, site fees & shipping cost.
Since I sell the same items at the same prices all the time I just copy/paste a previous sale and change the date.
I have it set up with formulas to automatically tell me the following:
Total price including shipping price.
Total fees including item cost, site fees & postage.
Total net profit
Monthly profit
Yearly profit
Monthly profit percentage (so I can see my best months of the year at the end of the year)
Total items sold
Total types of items (Example: 100 Item A's & 75 Item B's)
Total profit of each style of item I sell
Average profit per style of item
Once you get the spread sheet built with the formulas It is really easy.
The built in formulas are the big trick in a good spreadsheet.
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Calculating Monthly Profits 
