I just discovered that there is a twitter account for the NPC Harold which tells everyone which items have been newly offered at Harold. The twitter account can be found here: http://twitter.com/stendhaltrade
I think this is a very good idea and and it lead to the following idea:
I think it would be a good idea to add an overview of what Harold is currently offering to the webpage. This would allow players to easily overview the offers without going to the NPC themselves or creating a separate character standing next to Harold just to check the offers from time to time. They would only have to visit him if they actually want to make an offer or buy something. Also this page could contain the offers made by the owner of a logged in account. Furthermore the page could include pictures of the items and be linked to the item database so that everyone can see their stats immediately without having to look the item up separately. It would immensely increase the comfort of using Harold as a trader. It would also increase the awareness that there is an NPC like Harold who trades things for you and because it would be easier for new players to find out about him if he is mentioned on the website.
So more players would use this feature because it's more comfortable to use and easier to learn about and the variety of items offered at Harold would increase. So the sense of having a NPC like Harold also increses.
More players using Harold leads to more trading between players which means more involvement of the players in the economy of the game. Which in my opinion is a good thing.