It all depends on wich Server you play and with wich Version.
There are also Anti Hack Programms installed on Servers. (Mangos has a rly good one)
Means you cant buy an item from a Vendor if he doesnt sell the item.
Or you cant cast a Spell from a Character if he doesent have it.
-But you can change a spell (specially Druid) and heal you while shapeshifted. (Char has the Spell so it works)
Also you can learn Weapon Skills ( if you know how to make it), all because the Trainer has the Skills. So Mages can wear 2H Swords and so on