How to stop overselling TCG singles across TCGplayer, Mana Pool and your counter

If you sell singles at the counter, on your own site and on one or two marketplaces, you've probably had the call: a customer bought a card online that already left in someone's binder. Each miss costs you a refund, a marketplace penalty or a seller-rating hit, and the time to find out what happened.

Owners describe sync tools that work "about 90% of the time." The other 10% is where the fees come from. Here's what actually causes oversells and the rules that stop them.

Why oversells happen

• Two or more stock counts. Your POS has one number, the marketplace has another, and a sync job copies between them. Anything sold between syncs is sold twice.
• Relative updates. "Subtract one" messages get lost or applied twice. After a few weeks the numbers drift and nobody knows which is right.
• Carts that don't hold stock. A card sitting in a parked sale, an open tab or a preorder allocation still looks available online.
• Last-copy races. Two channels sell the final copy within seconds of each other.

The rules that prevent it

• Keep one stock ledger. Every sale, receipt, trade-in and adjustment is a movement in one place. Marketplaces show a copy of that number; they never own it.
• Count holds as unavailable. Available = on hand − parked carts − open tabs − allocated preorders − unpaid web orders.
• Push absolute quantities, not deltas. Tell each marketplace "you have 2," not "subtract 1." A repeated push can't drift.
• Keep a small buffer for hot cards. Holding back one copy from marketplaces for cards that move fast removes most last-copy races.
• Import each marketplace order line exactly once. A unique order + line ID means re-running a sync never double-counts a sale.
• Send shortfalls to a queue, not to negative stock. When a marketplace order arrives for a card you no longer have, flag it as oversold so staff can cancel or substitute quickly, before the marketplace penalizes you.

A quick self-check

Pick five cards at random. Compare the count in your POS, on each marketplace and on the shelf. If they disagree, look at when each was last updated and whether any are sitting in a parked sale or tab. That tells you which of the causes above is costing you.

How TCGCORE handles it

TCGCORE is being built around a single inventory ledger. Parked sales, tabs, preorders and web orders hold stock, and marketplace sync pushes absolute available quantities with an optional keep-back. Each marketplace order line imports once, and short lines land in an oversold queue for staff. It's in early access. Join the list if you want to help test it.