Why marketplace item is still in stock after I buy the last one?

There is a marketplace item that has 1 in stock. I have purchased that 1 copy twice now and it’s still in stock. It seems like the seller has some kind of reserve inventory from which a quantity of 1 is instantly added every time the item sells out, without it ever displaying “out of stock”. Can anyone tell me what is happening / what selling feature this seller is using to do this?