I think a percentage would work fine for min/max prices. As Createure mentioned prices would still be able to rise and fall with the overall market just not with wild swings. It would keep any single trade (or group of them) from having a drastic effect on the market.
A lot of players make trades without regard to distance. They only look at the price. While they may have saved/made a bunch of gold they may have their vans tied up for days. Having the prices within a smaller range would give players the opportunity to have more purchase options at more reasonable prices. This would help them to be more effective traders by giving them the opportunity to select trades based more on distance than price.
I let a lot of basic resources go to waste because their are no offers at a reasonable price and distance. I do not want my vans tied up for more than they need to be.
I am having a hard time trying to find a downside to this. Those players who play the market for gold would still be able to do so. Although I don't like this practice I think it should be allowed to continue as, if anything, it helps stabilize the overall market by keeping products available.
Sun ~ I think bidding is a bad idea. It seems like there would be a lot of opportunity for abuse. It would also leave some part of the world out of every deal since this is a world wide game. I see no reason to not let supply and demand dictate the prices of any new/special commodities as with all of our other resources.
While I am at it... a couple of further suggestions...
A min and max button could be added near the price entry box to facilitate those people wanting to be on one end or the other.
Insure programming to keep players from entering impossible trades. There was one today for 300k of something. There is no way to complete this trade with the current 70 van/210k limit. On the other side a popup warning would be a good idea if you try to accept a trade that would put you in jeopardy of running out of a required resource.
I guess it is three cents worth now...
Prom