Ludwig snare drums like any musical instrument enjoy a strong demand in the resale marketplace. Collectors will snap up the older versions, the newer versions will go directly to use, with students and musicians. So what do you look for? What is important?
As always condition is going to be one of the strongest determinants of value. On drums, especially the older ones you can get away with the drum needing new heads (the part that you hit with the sticks). On the newer ones unless it is very inexpensive I would shy away from the ones that need repairs. The drums should be free of any major dents or damage. Cases can add value for students that will need to have the ability to transport them from place to place.
Value? Good used, playable drums will sell easily in the $75.00 to 150.00 range. Older drums can sell for prices reaching the thousands.