Small correction: it's the MMSI number and not IMSI. Very easy to get online from BoatUS. Fill out the form and you have your unique number. Once programmed it can only be reset in the factory. There are very few handheld radios with DSC on the market but they are worth it in case of an emergency.
Resale shouldn't be impacted as it is easy to transfer the number from the BoatUS site to someone else. Change the username and password and give the username and password to the buyer. Bang: it's their number now with all their info.They buy a radio with DSC because they want DSC so nothing is lost. If they don't know what DSC is they won't pay for it anyhow.
http://www.boatus.com/mmsi/default.asp
One word of caution: if you take your radio onto a different boat for a longer trip change the info you registered! They may search for the wrong boat in case you use the emergency button...
Icom IC-M92D, (IC-M91D Europe)
Standard Horizon HX85, HX471S
LOWRANCE LHR-80
Uniden MHS235
to name a few (if not all)