One of the questions I hear most often from eBay sellers is how to value inventory for purposes of preparing their tax return, especially if it was purchased at a garage sale, or if you used the item before you sold it on eBay.
For new items that you purchase for inventory, make sure you keep all of your receipts. In addition, you might want to keep a spreadsheet with a description of the item purchased, date, and the purchase price, including shipping costs.
For items that you purchase from a garage sale or thrift store, you may not get an itemized receipt from the seller. So, I would encourage you to write up a receipt (carry a small notepad with you while garage sale shopping or thrift store shopping), while you are still at the garage sale or thrift store. Record a description of the items purchased, date, amount paid, and the location. Ask the seller to sign the receipt you wrote up.
The hardest inventory to value is inventory that you used for personal use before you sold it on eBay, such as clothes you bought for your children that they have outgrown. Before you sell these items on eBay, you should research similar items to see what they have sold...