if you add 10 dvds (same kind)... and theyre up for 13 days (or less), all are sold except for 1 (or more) and you decide to add another 10 for same price and all, itll add those 10 new dvds of the same kind to the others, theyll automatically share the same "On market time" as the old ones, meaning theyll get removed after only one day on the market since its "been 14 days"

(it can be avoided by listing at a different price though)