"Ik moet postzegels kopen."
Translation:I need to buy stamps.
Also don't forget to mention all kind of zegels linked to zegeltjes plakken which is (or used to be?) almost a national sport. For instance at supermarkets you get zegeltjes for every x amount of money spent. Then you have to stick these zegeltjes on a spaarkaart and if you hand in a full spaarkaart you get e.g. a discount or some products for free.