75gpd is a standard for RO filters. These generally have a 98%+ rejection rate. Trust me when I say, 75gpd is NOT a lot of water. It is a very slow stream. The higher the rejection rate, the less DI resin that is used up to clean up the rest of the water.
Ebay units are generally garbage; a mix match of crap and junk in a neat package. Unless you know what you are looking at, you would be better off going elsewhere.
Russ from
www.buckeyefieldsupply.com is a local by me, is incredibly helpful and will be happy to set you up with what you need; not sell you stuff you don't.