[Spirit]

So, if you go to the store and you find a really nice shirt that you want really bad to wear to an upcoming party and the price tag says $1.00, so you think, "Great awesome deal!!!" and go buy it, the sales clerk sells it to you for that price, but in actuality the shirt should have been priced at $100.00 - you gonna feel guilty? Are you gonna return the shirt to the store and say, "Ummm.... charge me another $99.00, please."
Not me. I'm gonna wear the shirt. In marketing, you win some and you lose some. It is up to the person selling the merchandise to make sure that they price it correctly. I can tell you from being a manager of a retail store, that if a clerk mismarks an item, you are bound to sell it as priced.
Not to mention that with the way weapons are not selling, he doesn't know for sure that they did not mark it at that price intentionally. He should not feel guity about finding a great weapon at a really good price.
I bought a level 60 staff last night (green, with decent stats) for only 50s. I then saw someone selling the exact same weapon for 150s. Hmm.... level 60 weapons (white) sell from the NPC at around 260s. I found a really great deal. It was a free drop, so the person I bought it from made 50s, could they have made more? Probably. Would I have bought it for 150s? Nope. I would have waited to see if I could get one to drop for me.
Maybe that person had already tried selling it for more and could not get it to sell.