9 times out of 10 buying a used RO/DI is not a good idea. You can bet it will need new replacement filters, DI resin and the membrane will be on its last leg. That alone will set you back anywhere from $75 to over $100 for good quality replacements. You can buy a new reef quality RO/DI including a 1 micron absolute rated sediment filter, 1 micron 12,000 gallon carbon block, a specially treated and batch tested high rejection rate 90 GPD RO membrane, full size refillable DI filled with 20 oz of custom blended fresh mixed bed resin, a capillary tube flow restrictor you adjust to suit your own water conditions and an inline presure gauge for only $130 and not need any replacements or modifications here: Refurbished 90-GPD RODI System
Buying a used skimmer is a god way to go, The last several I have owned were used but just like new once they were soaked in a little vinegar and it saved me a mint. They don't have consumables so as long as the motor is good and they don't have any cracks or leaks you are good.