In Guild Wars, you buy skills once, at that even unlocks them for heroes on the account. Guild Wars has the rather odd design feature that not every skill trainer has the same skills. If you want to get all the skills for your class, you need to tour around to different cities. Not all the cities and outposts have skill trainers. Secondary profession changers might be somewhere else entirely, and you need to have swapped to that profession to buy the skills. Some of the trainers are in explorable zones, so you cannot change your build after picking up the new skills.
Is there anything at all to recommend this design for skill training? I’m down with picking up elite skills from bosses, but aside from having a few early trainers with only a few options, which you could also do by hiding some skills until level X, what benefit is there to having this random spread of skills and trainers across the zones? And campaigns, because not all the basic skills are in all the campaigns.