When you sell personal property, you owe capital gains tax if you sell the items for more than you paid for them. If you do this more than casually, you need to keep accurate business records of all the items that you sell, their original purchase price, the original date of purchase, the date you sold, and the price you received when you sold them. All of these records need to be available to show the IRS in case you are audited.
Going forward, I can't see any good way for you to put the sales on your husband's business account. You really should have a separate account with PayPal, or square, or someone else to handle the transaction. Selling personal property is not really compatible with other kinds of business activities. For example, if you entered your personal property in your husband's business records, you might end up showing a higher cost of goods sold if you put in the price you paid, and this would create a business loss that is not allowable or real.
Looking backwards, the IRS does not have to give you any deductions or cost basis that you can't prove with accurate records. There is at least one tax court case involving a woman who claimed to have sold used personal property on eBay, making many thousands of dollars. She claimed everything she sold was used and sold at a loss, but she couldn't prove it and the IRS assessed tax and the tax court ruled in favor of the IRS.
You will need to reconstruct your records as best you can. For example, you might be able to show that if your husband's business is Monday through Friday, that all the Saturday sales transactions were from a garage sale. Do the best you can to document the identity of the items sold, the price you originally paid, the selling date, and the sales price that you received. A spreadsheet would be helpful. But understand that anything that you can't prove with your own records, is likely going to result in a higher tax assessment for your husband's business.
