Aspects: Zen-cart with normal database and SKU. eBay store with fixed price selling and SKU. No auction listings on eBay and no need for import/export product detail. Need is quantity change sync (reduction or update of quantity) to prevent oversell on eBay and Zen-Cart.
For SKU existing in Zen-Cart DB -and- SKU on eBay store that match, eBay will be master DB with quantity record. (It would be acceptable to maintain seperate lookup table or DB for matching SKU numbers if desired) Sale on eBay updates SKU quantity as normal eBay operation and decreases Zen-Cart quantity. Sale on Zen-Cart will reduce SKU on Zen-Cart as normal and on eBay to reflect correct quantity. Listing will be ended on eBay if Zen-Cart sale causes inventory SKU quantity to be zero. Additional count of SKU for quantity increase on eBay will be done manually by owner but will sync to Zen-Cart for Zen-Cart quantity update. Sync time period and API calls to be frequent to prevent mixed states.
Item with SKU ABC exists on eBay and Zen-Cart with quantity 50.
Ebay sale of 1 - normal eBay operation resulting in quantity 49 available to sell indicated on eBay and Zen-Cart.
Additional Zen-cart sale of 10 cause decrease of Zen-Cart as normal -and- eBay quantity to reflect current quantity 39.
*Additional eBay sale of 39 count cause eBay to end listing as normal -and- Zen-Cart update to reflect zero quantity
*Additional Zen-Cart sale of 39 count cause Zen-Cart to have zero quantity as normal -and- eBay to end listing.
Seller repurchases SKU at later time, relists SKU via eBay. Software discovers SKU exists again with quantity and updates Zen-Cart.