Building a Resale or Consignment Marketplace on Shopify

I learned this through experience while building Kick Game. Running a resale business exposes gaps very quickly. Items come from many sellers, at different prices, through different locations and workflows. They do not behave like standard retail stock, and traditional ecommerce tools cannot model that complexity.

Those lessons became the foundation for Zelph. Not as a follow-on from anything old, but as a completely fresh system built from the ground up using a modern multi tenant architecture. Resale needed an operational engine beneath Shopify. A platform that understands marketplace logic and fits directly into Shopify's workflow without fighting against it.

Shopify's Biggest Limitation

The biggest limitation is simple to describe.

Shopify only supports one sale price and one cost price per variant.

In resale this simply does not work.

Real resale operations have:

• multiple sellers listing the same size at different prices
• owned stock and consignment stock with different costs
• different prices across locations
• different fulfilment rules
• different seller behaviours
• inventory that behaves like an order book

Shopify cannot represent this because it only understands one price per variant.

This is why queuing becomes essential.

Queuing Explained Properly

Resale requires multiple inventory lines behind a single variant.

In Zelph, one Shopify variant can contain many underlying entries. Each entry can have its own seller, cost, sale price or location.

Shopify cannot publish multiple prices at once, so Zelph uses a queue to decide what Shopify should show.

The correct definition is:

• Queue position 1 is the inventory line Shopify is allowed to see and sell.
• Everything behind position 1 is valid inventory but not surfaced yet.
• When position 1 sells, the next eligible entry moves into position 1.

Queuing is not about "sell readiness". It is about resolving price conflicts.

Most queued inventory exists because:

• different sellers set different prices
• different stock types have different costs
• different locations have different pricing strategies

Without a queue, you would be forced to create duplicate Shopify products, which is operationally impossible at scale.

Remote Sell and Consignment Depend Entirely on Queuing

Remote Sell

A seller lists an item they still hold. It becomes sellable immediately. When it sells, the merchant chooses how the order is fulfilled.

Shopify only ever sells queue position 1. This ensures the correct price is presented.

Consignment to Warehouse

A seller ships or drops off the item first. Merchant authenticates it. If the item passes, it enters the queue. If it fails, it never goes live.

Queueing handles which authenticated item should appear in Shopify when many exist behind the same variant.

Authentication Does Not Determine Sellability

It Determines the Fulfilment Path

This point matters, because most resale systems get it wrong.

In remote sell, the item is immediately sellable when listed. Authentication only takes place after the sale, depending on the fulfilment route chosen.

After a sale the merchant chooses:

1. Authenticate at the warehouse

• Zelph automatically generates a shipping label.
• Seller ships the item in.
• Merchant authenticates.
• If it passes: seller is paid and the item is shipped to the customer.
• If it fails: the item is returned to the seller with a penalty and Zelph can automatically source a replacement item for the buyer.

2. Allow a trusted seller to ship direct to customer

No authentication is required for trusted sellers. Seller fulfils the order directly.

For items shipped to warehouse before sale

Authentication happens first. If passed, the item enters the queue. If failed, it is returned to the seller with a penalty.

Authentication governs the fulfilment flow. It does not determine whether the item is sellable.

Trusted Sellers

Fulfilment optimisation, not queue priority

Trusted sellers do not get moved to the front of the queue.

Trusted seller status only affects the fulfilment path:

• they can ship directly to customers
• they can bypass warehouse authentication
• they reduce workload for the merchant

Queue order is still based on price, location, stock type and merchant rules.

Drop Offs

A new intake option introduced with Zelph

Drop offs allow sellers to bring items directly to a warehouse or retail store rather than shipping them.

Drop offs reduce costs and speed up intake. The drop off method does not influence queue position. It simply provides a more efficient intake route.

Zelph Lookup

One click product creation for resale teams

One of the biggest time drains in resale is product creation. Teams spend hours manually finding titles, descriptions, barcodes, images and technical data.

Zelph Lookup removes this completely.

Enter a style code or paste a URL and Zelph automatically builds the entire product record:

• title
• description
• images
• variants
• barcodes
• HS codes
• country of origin

A task that normally takes 10 minutes is reduced to a single click.

Queue Management at Scale Is Technically Hard

And even harder in a multi tenant system

It is one thing to manage a queue in a small system. It is a completely different challenge when:

• prices are changing constantly
• sellers are adding and removing inventory
• items are moving locations
• orders are being fulfilled every second
• thousands of Shopify webhooks are coming in
• staff are editing Shopify directly
• channels update asynchronously
• multiple tenants are running independently

Every one of these events can change the correct queue order.

Queue recalculation must work in real time without race conditions or drift. Doing this for a single merchant is already complex. Doing it across many merchants simultaneously is extremely challenging.

Zelph was engineered specifically to handle this scale. The queue engine, webhook system and multi tenant architecture were designed together to keep everything synchronised.

Why Two Way Sync With Shopify Was Non Negotiable

Shopify is constantly updated by staff, apps and sales channels. If your platform does not ingest these changes, the queue instantly becomes incorrect.

Most systems only push data to Shopify. Zelph absorbs Shopify changes and reconciles them immediately.

This prevents mismatches and keeps the operational truth consistent.

Why Zelph Had To Be Built From Scratch

Resale requires:

• queued inventory
• multi state lifecycles
• authentication workflows
• trusted seller fulfilment
• drop offs
• unique unit tracking
• real time Shopify sync
• queue recalculation at scale
• multi tenant isolation
• product lookup automation

These are core architectural requirements. They cannot be retrofitted into an existing system.

Zelph was built to be the operational engine that resale and consignment businesses need under Shopify.

Final Word

Building Kick Game taught me what resale actually demands. Shopify is the best storefront in ecommerce, but resale needs a system underneath it that understands marketplace pricing, sellers, item level logic and the constant changes flowing through the business.

That is why we built Zelph from scratch. So the next generation of resale and consignment businesses can finally run properly on Shopify.