Generally, unless something is truly rare, and it can be well documented, or certified by Leica, then probably little chance of gain. One exception might ba a Leica rangefinder used by a professional photographer, but only if that photographer gains some notoriety.
All III series bodies are generally good classic style user cameras. The IIIG is one exception, but prices are already so high that they might actually drop in price. This market is sort of like buying a Ferrari, just on a cheaper scale. You would be better off actually using one, rather than speculating.