EBAY has "easy to read" 1-page report that shows YTD totals for (orders, refunds, EBAY fees, shipping expense). Go to PAYMENTS, REPORTS (select LAST YEAR). This provides most of the information you will need to tax time.
GROSS INCOME (this is the amount you report to IRS)
-Refunds
-EBAY fees
-Shipping expense
-Other expense (cost of goods, mileage, packing materials, office supplies)
NET TAXABLE INCOME
You just need to create spreadsheet to track your inventory "your cost of goods". Your spreadsheet should include (date you listed item, your cost, description, date sold, refunded merchandise, date resold). You keep the same inventory sheet "forever" (just keep adding or subtracting).
$3,000 BEGINNING INVENTORY (Jan. 1)
+$1,000 New Inventory Added
- $500 Inventory Sold
+ $300 Returned Merchandise (returned to stock)
$3,800 ENDING INVENTORY (Dec. 31)
You should create a spreadsheet to track miscellaneous expenses (mileage, packing materials, office supplies). Be sure to keep all your receipts to prove your deductions.