well ur description is very short
Store them ? for what purpuse? to rebuild ur city?
About spent FPs - i think it should be bit diffrent - if a player is selling a GB all contributed FPs should go back to their owners, but i also know players may use to gain FPs so there shoud be a limit: once per GB. So for instance i build notredame, now im selling it - all FPs goes back to rightfol owners, but then im placing it again leving and selling - 2nd time FPs wont go back.