How to by-pass the unbalanced trades limitation

Don't take me wrong, I am not looking for a loophole for the gold selling sites. However, 3-30K limitation is quite severe and there are legitimate ways to sidestep it when trading with your friends. Take a look at Merch Gwyar's note about it.
To add to this approach I also suggest to use Kegs of Chef's Delight beer. Each keg costs 60K in the General Exchange. A few days of brewing and you and your friends can exchange quite expensive items.