@Bislo1

Yes, you are right, the ones selling you for 3£ is the ones who are nuts, but aren’t these prices [Buy] prices? i mean, you can go to the island, find the right shop and then buy a diamond for 3£ – THIS is what i do not understand, if you have a median of 9,45£. shouldn’t the lowest price be more close to the median?

No, those are [sell] prices.

I’m not trying to be smart and I don’t know how things have developed, but it sure seems like someone thought to themselves that if winning all the competition means lowering prices to almost nothing, then that’s what we will do. I can see this on several of the items listed in here..

The actual process was gradual. I think I was the one who moved from $9 to $7, because I had too much in stock and someone kept selling them to me at $3.

I think this idea of listing items is a great thing. Easy for everyone to see where to get the best deals, if they can find the shop they are located in.
Then you might appreciate:
https://dl.dropboxusercontent.com/u/37084051/market.zip (raw market data in CSV format).
https://dl.dropboxusercontent.com/u/37084051/market.html (outdated marked data with locations & map links)