As a buyer I found that the key is to find a seller who specialises in items that were originally good quality and high ticket prices. For example there's a UK company called Rohan who make outdoor/adventure/travel clothing, it's great gear but eye watering prices. However on Ebay it's relatively easy to find sellers of this gear and, because the stuff is good quality, older items are still perfectly servicable ( even waterproofs ). Also I think that the better quality manufacturers will have more consistent sizing which makes buying easier.

Personally I prefer Rohan's older designs as I think that they're more practical which is even better because they're cheaper! These days all my walking gear, other than boots, comes from Ebay.