ro units

Hi new to reef keeping, at the moment I collect ro water from my lfs , I
would like an ro unit but have little room and a wife who is not keen on
having it set up perm. I have been told they must be kept running constantly
, yet I see on ebay units for sale, good prices and say can they be run
intermittently. how do you go about storage when not in use and does it
damage them to use like this?
thanks Paul, Beds UK