Cost basis is what an item truly cost you to acquire: the purchase price plus the costs of getting it — auction buyer's premium, sales tax, shipping, refurbishment. When the item sells, profit is the sale proceeds minus this basis, so an accurate basis is what makes your reported gain (and the tax on it) correct.
Recording only the sticker price understates the basis and overstates your profit, meaning you pay tax on money you never made. For resellers especially, the difference between the hammer price and the all-in cost basis is exactly the gap that fees like the buyer's premium create.
Cost basis is the per-item foundation of cost of goods sold. For one-of-a-kind goods it pairs naturally with the specific-identification method, where each item carries its own basis from acquisition through to the sale that finally recognizes it as a cost.
Related terms
COGS (Cost of Goods Sold)
The total cost of materials, labor, and overhead directly tied to producing the goods you sell. Tracked automatically through purchases, recipes, and production runs.
Specific Identification
An inventory accounting method that tracks the actual cost of each individual item, rather than averaging or formula-based costs — ideal for one-of-a-kind goods.
Buyer's Premium
A percentage fee an auction house adds on top of the winning bid (the hammer price) that the buyer must pay, raising the real cost of every lot won.
Comps (Comparables)
Recent sale prices of comparable items, used to estimate what a one-of-a-kind or vintage piece is worth before buying or listing it.