In 2018 a Korean stamp dealer sent me several envelopes covered in older stamps. All stamps published by the (South) Korean postal services are still valid, even including the hwan stamps printed before 1962 and the old won values from the 1940s and 1950s as long as they are correctly recalculated into new won.
That last trick would be rather expensive, but using much older stamps for postage instead as items for sale to collectors is a very common trick and I have seen it often on envelopes from the rest of the world. However, since most people don’t regularly receive post from Korea I wanted to show these examples. They prove that catalogue prices in the Korean stamp catalogue are just as overvalued as they are in most catalogues.