The till is instant; eBay is checked
A counter sale reaches eBay in about a second; the return trip takes up to two minutes.
Lightspeed reports a sale the moment it happens, so the eBay listing drops to match in about a second. eBay announces nothing, so we ask it every two minutes, and an eBay sale reaches the register on the next check.
Two minutes matters only at speed. An item selling twenty a day sells one every seventy minutes, and a two-minute window on that is a fraction of a unit. Hold one back on anything genuinely fast and the window stops being a risk.