Well i'd think that the problem is not for an item that works for ur class but for an item that does not work at all for u. Then it should be better maybe to explain for what class is this item. For instance i wanted to buy some items to equip a ranger merc but since i have never had a ranger merc i cannot recognise what item i should buy for him/her. Ofcourse i can understand that throwing knives or a bow is for a ranger. But what about helmets? Or Gloves? etc... How do i know which of the ones for sale are for a ranger and not for a warrior?
I know that if you are not that particular class its not easy - but the problem remains that most of the things can be equipped by 2 or more classes !
The best way to remember is that things out of metal are mostly for warriors, especially if it says plate on them - iron plate armor, gloves, helmet, etc. The rangers are a bit trickier - in tier 2, they cant wear metal until they reach level 15, but after that they wear same equipment as warriors. In tier 3, most of the medium armors for them are made of metal but they say - chainmail, not plate - this applies to helmet, gloves, etc. Mystics always or almost always wear stuff made out of leather, they are the most restricted.