Stock counts for a sneaker shop selling on marketplaces

Your company sells sneakers on its own site and on 3 online marketplaces. The warehouse system holds the true count for each of 20,000 SKUs. A SKU is one model in one size. Each marketplace keeps its own copy of each count and sells until its copy reaches 0.

The warehouse system emits an event for every change: a sale on any channel, a return, or a delivery from a supplier. That is about 60,000 events a day, and up to 3,000 a minute when a limited release goes on sale. Each marketplace's API sets the counts of up to 200 SKUs in one call, and allows 1 call a second.

When a SKU sells out, every marketplace should stop selling it within 2 minutes.

When does your service call each marketplace, and what does each call carry?