I suggest wave accounting, it's free and easy, web based, and does real double entry bookkeeping. Like quickbooks online, but without the monthly fee.
If you use paypal, they will send you a 1099-K for your total receipts if you cross a certain threshold. IDK what it is, but the one year I received one it was around $20K of receipts. Of you'll want to be sure to expense your ebay and paypal fees, shipping costs, mileage to post office, etc.
What are you selling? Just random stuff around the house? Or are you maintaining some kind of inventory?
(Even if you are cash basis, you need to keep track of inventory.)
I was involved with a direct sales company and sold some of it on ebay. I don't do it anymore, but that's where my info is coming from. I didn't actually use wave, it was something else that go daddy purchased, and it is now called godaddy accounting (or something similar). It interfaced directly with either paypal or ebay, so most of it was pretty automated. But my husband uses wave for his small business, and I like it (as an accountant).