Selling tangible personal property is a capital transaction reported on form 8489 and schedule D. If you sell for more than your cost basis, you have a capital gain. If you sell for less than your basis, you have a capital loss. However, capital losses on personal property are not deductible and do not offset your gains. So if you sell 10 items with a combined loss of $1000 and one item with a gain of $10, you have a $10 capital gain.
You need a spreadsheet listing your items with a description, date purchased, cost when new, date sold, and selling price. If you don't have receipts, make your best guess and hope you aren't audited. Without proof of your cost, the IRS would be allowed to disallow your costs and declare the entire proceeds as taxable income. A listing made at the time of sale is better than nothing, but not as good as receipts.
Basically, do the best you can. The more complete and businesslike your records, the more the likely auditor is to cut you some slack if some of your items are less-well documented. Keeping your listings may help as well, if you are listing items as used, that will help to establish they were your previously purchased used items. Without good records, you have to hope not to get unlucky, and do the best you can.
