This pair is checked more than it is told
Squarespace announces orders. Everything else here we go and look for.
Three of the four directions on this pair run on a check rather than an announcement. Squarespace tells us the moment an order is placed, and that is the one live signal. A stock figure you edit by hand inside Squarespace comes with no announcement, so we look every couple of minutes. eBay announces nothing at all, for stock or for orders, so it sits on the same short timer.
None of that makes the pair unworkable, but it does mean you should not expect either side to react within seconds to a number you typed yourself. If you have just counted a shelf and corrected the figure in Squarespace, give it a couple of minutes before you expect the eBay listing to agree. Knowing that up front beats refreshing a page and assuming the connection has broken.
The practical answer is headroom. Keep a unit or two spare on the items that sell fastest and the timer stops mattering, because the window in which the two sides can disagree is only ever a couple of minutes wide. Where it does matter is a one-off item you cannot afford to oversell, and for those the safer habit is to end the eBay listing yourself.