Re: How much is reasonable?
I'm not sure what you might get currently for the Flips, especially with them going on Cottonbabies clearance currently. CB has covers for $9 and organic inserts for 3/$15. I think that might also be why the pp's Bumgenius 3.0/4.0 aren't selling, as they have been on CB clearance as well.
I don't see Weehuggers very often on FSOT! But I think they you could get a fair price for them.
I am actually interested in your Weehuggers bamboo inserts. I haven't seen any used for sale, so not sure what you are wanting for them. But, PM me if you are willing to discuss.