I've heard of this before - there's a particular guy who brought them in (I forget his name) and he maintains that the expiry date relates to how long the capsule has been actually
used for instead of how long it's been since it was manufactured. So he put a whole bunch of stickers over the expiry dates stamped on the seats when he originally sold them.
From a scientific perspective I can see where he's coming from, since if it sits in a box for a couple of years it's not like the plastic is being exposed to light and therefore degrading, so it's
probable that the seat is fine up to the sticker date. Some people don't trust that though... I guess it's up to the purchaser when you go to sell them. If they're scientifically minded then you won't have a problem but if they're not then they might think it's a bit dodgy and I can see why