『This product is out of stock.』은 "이 상품은 품절되었습니다."라는 뜻이에요.
발음: 디스 프로덕트 이즈 아웃 어브 스탁
'Sold out of stock' mashes two separate phrases ('sold out' and 'out of stock') together. Native speakers find it grammatically broken and confusing — they can't parse whether you mean the item was sold from somewhere or is simply unavailable.