@lilbabyplain;
The cost basis is defined as "the amount [money] of your investment in property for tax purposes". Basically, it's what you paid for the item (if you paid for it) or the item's fair market value at the time you acquired it (if you didn't pay money or currency for it; as in you inherited it, or it was part of an exchange or barter). This is defined in IRS Publication 551, Basis of Assets:
https://www.irs.gov/forms-pubs/about-publication-551
Your cost of goods sold is what it sounds like; it would include the cost basis, plus any other costs you might have paid for the items, such as a buyer's premium if you bought them at an auction, shipping cost, and the cost of repairs or improvements that you made to the item(s).
When you are selling your personal items, it depends if you are selling them as personal investments (i.e. capital assets) or as a business. Either approach is acceptable, as long as you are consistent and reasonable. Because you said that you have purchased items for resale, that means you are operating as a business, so that's how I am answering you. You need to be consistent, and business like, and take steps and make decisions that would increase your profits in the long run.
Whether you have receipts or not, you can document the fair market value of your personal possessions, and it is strongly advisable to make a spreadsheet to keep track of your cost basis in the items that you decide to sell. It is quite reasonable to use your personal possessions as inventory to build capital for your business, because you have to have money to start up and get your business going.
You can use the purchase price or the fair market value of your items when you acquired them, as the cost basis, but this is likely to result in your business losing a lot of money on paper. This is acceptable (as long as you are consistent) but if your business loses too much money, and if you are audited, then the auditor may decide that you are not really running a business activity and are not entitled to deduct business expenses.
You can lose money for a few years, because the IRS understands that businesses are likely to lose money in the startup phase. But, you need to be taking actions that will improve your profit, and you can't keep losing money forever, because that's not what a real business would do.
One approach that is often used by startup sellers, when they are operating as sole proprietors, is to transfer "ownership" of the items from their personal possession to their business possession when the item is sold, and to use the fair market value of the items at the time they sell as the cost basis: in other words, the sale price. As sole proprietors, they still own the item whether it's a personal possession or a business possession, and they report the profits from the business sales as income on their personal tax return, so this is completely above board for reporting taxes.
This way, the personal items are sold at a loss (because of the expenses of eBay fees, shipping, packaging materials, and so on) but the loss is much lower than if the original purchase price was used as the cost basis (assuming the items were sold at a loss). This way, you can use the money you receive from selling personal items to fund your business, and purchase items for resale that you can use to make a profit. This way, you can show that you are making good business decisions to improve your profit and loss. Make sure that you keep receipts and records to document the cost basis of things that you purchase for resale.
Whichever way you decide to compute the cost basis of your items, you need to be consistent, and you should work with a tax expert to make sure that your business model is correct and will be accepted if you are ever audited.
