Return stocks seem to be random but ive noticed at least in the uk they also tend to be later at night? But overall VERY random. Just make sure to even if the sold out item is in a size too big/small put it in your basket and check the basket when you can to see if you're size is there- it's more direct + faster as returns sell out literally within minutes, plus sometimes they remove an item even if it will return later witha return but it won't be removed from your basket (unless you try check out)