Honestly, a good way to do it is to make there be a time limit on when trading is unlocked in clans. You join a clan, you have to wait 2 weeks (maybe more) before you can trade anything between each other.
After the 2 week period is up, you can only trade, oh... what... 3 things per day to prevent possible problems? I don't know, but I know with a 2 week wait to get trading in a clan, it may be a good enough deterrent. Most of the gaming population nowadays is very impatient, and it could be a good start.
At the end of the day, I absolutely believe that trading in clans and clans alone should be the way to go. I don't know how many times something that could've been used by someone else in my clan that I didn't need ended up salvaged and it feels like such a waste. I understand that Blizz doesn't want to end up with sites selling items and whatnot, but there's got to be a way to make this work and make things hard on the illegal websites out there. I just don't know the perfect solution that will work to prevent the d2jsp sites.
Cause clans are so useful that you really want to be in your own clan...
I mean, if your system is implemented, there will be clans with like 100bots farming and the rest will be buyers (if there even is a limit on people in a clan, else it will be 1 big clan).
As I "don't know" that my clan members that "gives" me that WH, starmetal, tasker, ... is a bot, blizzard can not ban me.
Don't get me wrong, I still miss my AH and trading option, and would LOVE to have it back, but either keep it the way it is, or put it back like it was (even with RMAH, as it is safe, people that did before would still buy with money anyway, just less safe).