What is an assay?
An assay is the testing and certification of a metal's weight and purity. On modern bullion bars it usually arrives built in: the bar is sealed in a tamper-evident card bearing the refiner's stamp, serial number, weight, and fineness.
The assayer matters. Bars from refiners on recognized industry accreditation lists are accepted worldwide without question; bars from unknown refiners may be re-tested, at your expense, before anyone buys them.
Why does assay matter at resale?
Because buyers pay for certainty. A sealed, assayed bar from an accredited refiner can be sold on its face; an unsealed or uncertified bar invites skepticism, testing fees, and lower bids.
This is also why you should never break the seal on assayed packaging. The moment the card is opened, the bar reverts to 'trust me' status, and its liquidity goes with it.
How does assay show up in practice?
On a quote for bars, look for the refiner's name and 'in assay' or 'sealed in assay card.' For coins, government mints certify weight and purity directly, so no separate assay is needed, one reason coins trade so easily.