How do I sync stock between Etsy and eBay?

Connect both marketplaces and they share one pool: an Etsy sale updates eBay within seconds and the reverse, with each side’s quirks handled natively (Etsy’s variation model, eBay’s GTC listings and out-of-stock option). One-of-a-kind pieces are the acid test, quantity 1, never sold twice, the Etsy-eBay guide.