You are still allowed to organise with other players if you have something they want, but the point of the rule is that you're not allowed to, for example, sell something for a ridiculously low price to one player in return for another ridiculously low priced item in another world. The same goes with ridiculously overpriced items.
You also cannot sell something to someone cheaply, for say 5k, then have then sell it back for 10k and get a 5k profit out of it.