For this last year, use eBay's numbers that they gave you in your weekly and/or monthly reports. Estimate your cost of goods, mileage, etc as well. Plug all those numbers into your tax form with any others you can reasonably estimate and bob's yer uncle. Underestimate for this year. Better to have to pay a few extra dollars now than pay fines later. Although, the IRS really is not interested in you or me. And they are so overwhelmed and understaffed these days that they're only "going after" the most egregious of the big boys.
For this year, start a series of columns with unique names. Every time you buy something/sell something add it to that list separating each part of the purchase/sale into the various columns. If you do/buy/come across something new - add a new column.
Date / Mileage / Where / Cost of Goods / Sales Tax Paid / Had Lunch / Sold Locally / Tax Collected / Sold on eBay / eBay fee / etc . . . etc . . .
A spread sheet is made to handle stuff like this. But, at the VERY LEAST make a list on paper. Use several sheets if need be. But keep records.
