The register is instant; Etsy is on a timer
One direction corrects in a second, the other within five minutes.
Lightspeed tells us the moment the till rings, so when something sells at the counter the Etsy listing drops to match in about a second. Etsy doesn’t announce orders to anyone, so we check every five minutes, and an Etsy sale reaches the register on the next look.
What that asks of you is small and specific: on the handful of items that sell fastest, keep one unit of safety stock. A five-minute window on an item that sells twice a day is no window at all; on the market-day bestseller it’s worth the single held-back unit.