I HAVE A STUPID QUESTION, I WILL BE MOVING IN 6 MONTHS, BUT WANT TO BUY A RO/DI UNIT THAT IS PORTABLE AND, HERE IS THE STUPID PART, :iamwithst INEXPENSIVE. ANY HELP?
the only thing that is really "permanent" about these systems are the connections to your plumbing for the supply water and drain/waste water.
Make these connection in a way that's not permanent and you're good to go.
Be careful about moving a system that is full of water, and don't let the system go unused for too long.
As far as cheap/ebay - think long and hard about going that route if you are looking for inexpensive. No doubt you can get a system in hand inexpensively going that route, but there are literally hundreds of posts on boards you probably visit explaining the short-comings of that approach.