I only have on rilezu cel, and really, I didn't 
buy it with an eye to resale value (I'm not sure 
I've ever bought a cel with that in mind -- it 
does have an impact on my decision to let some 
go, though). It was expensive enough (a douga was 
included in the purchase) and dear enough to me 
that I really have no intent on letting it leave 
my collection at any point (even after I stop 
collecting). A second OVA has been announced for 
the show, so it's possible it'll gain new fans 
and potentially more value, but I'm not 
particularly concerned if it does...