retaining value yes, those lights have agreeable attributes that can be considered as perfect: concepts like the Surefire A2, Inova X5, etc.
gaining value is a prospect of demand. custom lights manufactured through small batch technology by modders on the forum may gain in value overtime as they are highly demanded during production or mass produced products discontinued over time.
The McLux-PD, Lunasol...Black aviator, PKEF, etc fit this category quite well
How much value is gained is relative to the trends in the market, in this case: the forum.
Selling a McLux to an unenlightened folk for $275 aint gonna happen, but selling it on here it might be able to get more for it