[Knil]

So I'm just jumping into conversation (which is probably not the smartest thing, but oh well) to give my 2 cents.

Guilt is something you should know you will feel at the time of decision. If it affects you, then talk to them at that time and let them know their mistake. If it doesn't, then carry on with your day. It's not really a question to ask, its just based on personal moral. If anything, you just started a pretty heated debate (which is an awesome read btw), and you will just end up going with your initial gut instinct.

Personally I wouldn't feel guilty because it's something we all do and have gone through at one point or another. Not to mention they will (hopefully) learn from their mistake when they notice a load of money not in their inventory, and be more careful when putting things up for sale next time. And they accidentally just helped out a lower level. If it happened to me, and I found out someone level 20 bought a nice green real cheap from me, I'd be happy I helped them out. Who knows, maybe fiesta karma will give you some amazing drop for selling it so underpriced...