If it helps at all, here's my perspective as a buyer. I will often keep a sold or out of stock item in my watch list for as long as I can, or is necessary, and not delete it right away. I've seen it enough times when the seller relists another one (or more) hours, days, weeks or a month later. If I had deleted it from the list when it did sell then I may not have known about the 'back in stock' / relisting. This doesn't just apply to new (or NOS) never used items - it applies to ones that are "used" as well. It just depends on what the item is.
Many of the things I've bought are used and are the one-of-a-kind sort of things, or harder to find, etc. (No, I'm not referring to collectibles.) You never know when someone else will have another one for sale and in what condition. I have learned to not trust or rely on any of ebay's notification systems to let me know of new matches, especially recently.
This brings up a question that I've been wondering about. When a seller has more than 1 of the same item, then why not just list that quantity in stock to begin? Why wait until after that 1 is sold before listing another same one? I know that this doesn't apply to all sellers. It's just that for some of the ones I come across, it's kind of annoying. Look at it this way: If it appears that you have only 1 to sell, then after that 1 is sold, a buyer like me may not bother to come back to see if you have more. If you listed the quantity you had to begin with, then I'm more likely to keep tabs on how many may be left, and therefore, more likely to look through the rest of your listings. Wouldn't you want to have potential buyers come back rather than just leaving after finding out that 1 has been sold?
I'm aware that the seller may not actually have additional stock on hand, though it may be on its way, - this isn't what I'm talking about. I'm referring to the ones that do seem to have that additional stock, as seems to be indicated by the speed of which the new listing is made.
