Sell a used sofa online for $300 and the taxable income from that sale is probably zero. Sell a vintage watch you bought decades ago for $80 and get $900, and you have an $820 taxable gain. The difference between those two outcomes is a single number the tax system calls cost basis: what the item cost you. Every dollar of basis you can document is a dollar the IRS never taxes, which makes basis tracking the highest-value paperwork habit an online seller can have.
The rules live in IRS Publication 551, Basis of Assets, and they are friendlier than most sellers assume. You are never taxed on the gross amount a buyer pays you. You are taxed, at most, on your profit: the sale price minus your basis and selling costs. The catch is that when you cannot prove basis, you can end up treated as if it were zero, and the whole sale price starts looking like gain.
Basis, in plain English
For something you bought, basis starts at what you paid, including sales tax, shipping to get it to you, and installation. If you improved the item in a lasting way (restored the furniture, replaced the guitar’s hardware), those costs add to basis. When you sell, your gain is the sale price minus selling expenses (platform fees, payment processing, postage you paid) minus that basis. If the result is positive, that is your taxable gain. If it is negative on a personal-use item, something you bought to use rather than as an investment, the loss is not deductible; the tax code treats wear-and-tear losses on personal property as personal consumption, not an investment loss.
That asymmetry defines most casual selling. The garage-sale reality is that almost everything sells for less than it cost new, which is why cleaning out the attic rarely creates taxable income. But “rarely” is not “never,” and the exceptions cluster exactly where online sellers live: collectibles, electronics that became rare, sneakers, trading cards, old toys in the box.
Gifts and inheritances follow different rules
Two special cases matter enormously for people selling family items. For a gift, you generally take the giver’s basis, known as carryover basis. If your mother gives you the china cabinet she bought for $500, your basis is $500 no matter what it is worth now (a special rule uses fair market value at the time of the gift for figuring a loss if the item had already declined in value).
Inherited property gets a far better deal: basis is generally stepped up to the item’s fair market value on the date of the previous owner’s death. Inherit a coin collection worth $8,000 at death and sell it for $8,200, and your taxable gain is $200, even if the original owner paid $300 for it decades ago. Inherited property also automatically counts as held long-term. For families liquidating an estate through online sales, getting a written appraisal or documented valuation near the date of death is the single most valuable tax document in the process.
Gains: how they are taxed when they happen
When a personal item does sell at a genuine profit, the gain is a capital gain. Held more than a year, it qualifies for long-term capital gains rates, which are lower than ordinary income rates for most people. Items the tax code classifies as collectibles (coins, stamps, art, antiques, gems, most memorabilia) have their own ceiling: long-term collectible gains are taxed at a maximum rate of 28 percent, as the IRS notes in Tax Topic 409. If you are reselling as an ongoing business (buying inventory to flip), the analysis changes entirely: that is ordinary business income on Schedule C, with inventory costs deducted as cost of goods sold rather than capital basis.
When the platform sends a 1099-K
Marketplaces and payment apps report gross payments on Form 1099-K once federal or state thresholds are met, and the form shows the full amount buyers paid, before fees, refunds, or any consideration of your basis. A 1099-K does not mean you owe tax on that gross number; it means the IRS received a copy and will expect your return to account for it. The IRS pages on understanding Form 1099-K and what to do with the form walk through reporting sales of personal items, including how to show gains and how to handle items sold at a loss so the gross figure is not taxed as if it were pure profit.
The recordkeeping system that makes it easy
You do not need software, just consistency. Keep a simple log with one line per item sold: what it is, when and where you got it, what it cost (or its value when inherited), what it sold for, and the fees and postage. Attach whatever evidence exists: receipts, credit card statements, order confirmation emails, the estate appraisal, even a dated photo of a price tag. For older items with no receipt, contemporaneous evidence of what such items cost when you acquired them (a catalog price, a bank statement from that month) is far better than nothing.
The habit pays off twice. It keeps you from overpaying tax on gross proceeds that were mostly your own money coming back to you, and it gives you a clean answer if the IRS ever matches a 1099-K against your return and asks about the difference. Basis is the rare tax concept that works entirely in your favor. The only way to lose its benefit is to not write it down.
