The difference IS in the filter cartridges. A e bay quality prefilter is not an absolute rated filter meaning it only has to trap a very small percentage or its rating and can pass the rest on. A good prefilter should be absolute rated meaning it traps everything larger than and including its micron rating.
An e-bay carbon may be granular activated carbon, which has absolutely no place in a RO/DI system, or it can be a no name Chinese knock off with no micron rating at all which is usually the case. Poor quality carbon has a very small capacity for chlorine and VOC removal, often as little as 30 total gallons, thats all water that passes through the system including waste so its 60 gallons of good water and 240 gallons of waste and its shot!
A good carbon block is a 0.5 or 0.6 micron near absolute rated block that treats 20,000 gallons of water or more.
An e-bay membrane is not usually a brand name 75 GPD like a Dow Filmtec but are usually Chinese or Taiwanese knock offs not even legal for sale in the US for drinking water. Other membranes commonly used are the Dow 100 GPD which again is not legal for drinking water but is certified by the ANSI/NSF for "Pool and Spa Use" not drinking water. Take a close look at some of their deceptive ads that claim 110 GPD, 125 GPD and others. No manufacturer makes a 110 or 125 GPD membrane.
The difference in membrane quality is vast, often from less than 85% up to Spectrapures guaranteed 98+% with the 75 GPDDow Filmtec coming in at 96-98% on average.
Di resin and filters are another big difference, look at the little e bay POS clear horizontal tubes with a bit of resin bobbing around in them, usually 6 oz or so. Laid horizontal and packed poorly the water channels or short circuits and is not even treated!Then look at a real DI filter which is inside a standard sized 10" vertical canister and holds 20 oz of resin with a true engineered bottom up flow pattern so all resin and water come into contact with each other.
With RO/DI you get what you pay for and cheap is not it.
