eBay can be great for some items AND from some sellers. You need to check the specs and prices against the big online retailers. I often find that I can a product for less money from DrsFosterandSmith, ThatPetPlace, BigAlsOnline or occasionaly MarineDepot.
I got a great RODI
faucet adapter on eBay from the filerdirect, Salifert kits from Tsunami, a detrivore kit from a Philly hobbyist who only has them as his population permits, and many other items too.