PSA: GMs are banning players for attendance.

Is it possible to be certain about the methods someone used to obtain a scroll, a mastery book or some piece of gear?
Some are obvious as you pointed out, but others aren't as much and in most cases (where it isn't obvious) it's impossible to know whether any given item is legit or not.
I agree with you though... it's easy to avoid obvious exploited stuff and ignorance isn't an excuse, but it's something that pisses me off.

The problem with that argument is, like most rules, it's just your responsibility to know. Nexon's perspective is that you just avoid what appears to be suspicious or too convenient. I'm not going to get into a debate about the morality of their perspective but the bottom line is, you have it, they can be as unreasonable as they want about it and delete your account for it. But for circumstances like that they normally remove the item with little to no compensation so I guess that's merciful.