Business bookkeeping is essential on Ebay or for any online business. The mistake I made when I got started was thinking my business would not grow so rapidly, but even though it started out just as selling a few things here and there, when I found my market niches, it started to grow. I wasn't really paying attention to the amount of additional work that would be caused by my finances, but I wish I did.
Every year at tax time I'd be in a panic because I literally had a weeks worth of work to do. I know what you're thinking. . .I should have kept up with this all along. Yes, I do agree. In the beginning of my Ebay business, that would have been possible without help. But now, forget it. I am not a numbers person because I don't like to do any type of accounting. So now I let OUTRIGHT, a convenient and affordable (free if you choose it to be) online business bookkeeping service do most of my work for me. Right now I am not paying anything for this service. I sell on several different sites. Outright combines all of the figures from those sites into different categories and shows the basic profit and loss statement. I will be upgrading soon because I need some additional help.
I would encourage you, if you are just starting out, to sign up for OUTRIGHT right at the start of your business. You will save yourself a whole lot of time.