especially new players ask about where they can sell or buy items from. Maybe it might be an idea to add an icon above the NPCs head to show that they are meant to do some items business there.
I think they can ask for 'offer'. If anything is for offer they will know otherwise not :)
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You make a good point but, a newcomer to the game may not be to motivated to approach every NPC in the game with the phrase "Offer". In the short time that I have been playing Stendhal, I was unaware of several NPC's capable of buying and selling items. Perhaps an additional icon is not such a bad idea.
I think this might be a good idea.
The wiki has a page defining which NPCs buy and sell what. But, no reason to not use every available resource. :)
Added to CVS 2013-10-2: http://stendhalgame.org/development/sourcelog/2013-10.html