Courses
 

Okay, perhaps it'd be cool if there's a sign at the end of each trading center, which opens up a list where you can see the courses. With courses I mean the value of several items.
For example:
diamond - 300 plat
inferno sword - 900 plat

It will take the average of table trades for certain items, but it will ignore certain trades - like 100 diamonds for nothing. This will make everybody who doesn't spend all his time at the trading tables and newbies aware of the prices of certain items.

While the idea is good, I doubt it could be done - imagine 2 or 3 players simply trading 25 diamonds for 500 each arround, raising the prize. Also, how would it judge the use of maps and other items as trading goods? Go by their shop-prize?