If you've built inventory for a typical e-commerce store, the core assumption is almost always: a SKU represents an interchangeable unit, and inventory is a quantity count against that SKU. Sell one, decrement the count. Restock, increment it. Any unit of SKU-X is identical to any other unit of SKU-X — that's what makes it a SKU in the first place.
A decent chunk of bullion inventory doesn't fit that model, and pretending it does creates real bugs.
Some units are genuinely fungible — and some aren't
A generic 1 oz gold coin from a large mint run is close to the classic fungible-SKU case: any unit is interchangeable with any other unit of the same product, year, and condition. Standard quantity-based inventory works fine here.
A serialized bar is a different object entirely. Per the LBMA's own Good Delivery marking specification, a compliant bar carries a serial number — up to 11 digits or characters — that has to match the number on its accompanying assay card exactly. That's not a SKU. That's closer to a VIN on a car, or a serial-tracked asset in an ERP system: each physical unit is a distinct entity with its own identity, not an interchangeable instance of a product type.
Model that as plain SKU-quantity inventory and you've thrown away information you can't get back — you know you have "3 units of 10 oz PAMP bars in stock," but you've lost which specific serial number a customer is actually going to receive, and whether that serial number's assay card is the one currently sitting in your safe or the one that shipped out last week.
A 1 kilogram silver bar with its own serial number, distinct from generic quantity-tracked coin inventory
Photo: Gilded Bullion
What this actually requires in the data model
A serialized item needs its own row, not a quantity field. Each physical bar or coin with a unique serial should be a distinct record with its own lifecycle (in stock → reserved → shipped), not a decrement against a shared count.
The serial number is a second key, not metadata. It has to be queryable and unique-constrained independently of your internal product/SKU ID, because a customer service question ("is serial #4471829 legitimate") comes in referencing the manufacturer's number, not your internal ID.
Fungible and serialized items can share a product page but not a storage model. A 1 oz gold coin listing might sell against a simple quantity count, while a 10 oz bar listing on the same site needs to reserve one specific serialized row per sale. Building one inventory abstraction that handles both cleanly is harder than it looks at first glance — a naive "everything is a SKU with a count" model works for one and silently breaks for the other.
Returns and buy-backs need to reunite an item with its original identity. If a customer sends back a serialized bar, the system needs to match it to its specific record, not just credit a generic quantity — this matters directly for verification workflows, since a returned item's serial should match what was actually shipped, not just the product type.
Where the two models have to coexist
This is the same distinction that makes something like a jewelry-grade alloy less traceable than an investment-grade bar with a named refiner — one is priced and tracked as a fungible commodity, the other as a discrete, individually verifiable asset. A real bullion catalog has both, often on the same page (buy 1, or buy 10, of a serialized bar), which means the inventory layer has to support per-unit identity without forcing every product into that heavier model unnecessarily.
Gilded Bullion's own catalog runs on exactly that split — our gold bar listings track serialized inventory per-unit where the product is serialized, while coin listings run on simpler quantity tracking. If you're modeling something similar, the mistake to avoid is treating "inventory" as one uniform concept across your whole catalog when the underlying items aren't actually uniform.
General information about data modeling and inventory systems, not investment or legal advice.
Alexander Sterling is the founder and CEO of Gilded Bullion, a US-based online dealer in gold, silver, platinum and palladium bullion.
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