Popular lenes will always remain popular - There is a very active market place here on this Forum (I have purchased a battery grip and a flash myslef) and of course there is always Ebay. Pentax lens in general I find are much harder to find in stock than say N and C lenses (supply and demand thing).
I don't think the 31mm ltd is a rare lens (i.e. realtivley easy to get) but rather a great lens so my bet is If you sell it for a
fair price then I am sure someone will scoop it up here.
fair price is purley subjective
Search the Market Place here to see if any of them have sold and what the selling price was.