depend on the model. if you having 27" monitor or above, there is a service tag behind the monitor, that you can punch in at dell online warranty to know the warranty detail. other smaller size monitor, will follow your official order/invoice date or receipt which is more benefit for end user than manufactured date.

my advise is ask the buyer to get back the original receipt for you is the best. just based on the sticker on the box, nobody can tell is the warranty still active or not (since 24" don't have service tag)? local or import unit? and also avoid getting ARB unit. unless the buyer had stated otherwise with cheaper price offer or you can send back to him if you want to claim warranty in future. BTW, how much is the price?