Neither of these can fulfil the other’s orders
That is how marketplaces work, and it changes what you should expect from a connection.
Etsy and eBay are both marketplaces, which means neither can be told to pick and post an order that came from the other. There is no arrangement in which an eBay sale gets fulfilled out of Etsy, or the reverse, and no tool will get round it, because the constraint belongs to the platforms rather than to us. Worth settling in your head before you go hunting for the setting.
What is left, and what actually causes the trouble, is the count. Both shops sell the same items off the same shelf and neither has any idea what the other has shifted. QuickSync sits between them, holds one figure and corrects each side from the other, so a sale on Etsy lowers what eBay will let a buyer take. Packing and posting stays yours. Knowing how many are left does not have to be.
Both sides are also on a timer, which is unusual for a pair. Neither platform volunteers anything: we read Etsy’s quantities every two minutes and fetch its orders every five, and we look at eBay every couple of minutes for stock and orders together. Neither wait is long, but with two polled sides it is sensible to keep a little headroom on your fastest movers.