In D2 you could trade anything. Because of this many people didnt play the game it self, they do not farmed, and only bought items for low price and sold for high price, till they were rich. In D3 you could not trade anything good, because all good things are fixed on the account. If you found in D3 anything very good but you didnt need this for your character or build, you do not have any benefit of it, because you cant trade it for anything you need. Because of this the idea to allow trading in general but limit some items for 1x trade is a very good idea, you talked about on blizzcon. So you can trade away the stuff you dont need.
There is another scenario you have to consider. Sometime you have played a build a long time and you want to try a diffrent build or character or you tried a new Build and after some days you know i dislike the playstyle. It would be nice if you could trade your items away. even when you already bought a item from someone. It will be hard to find a solution for this problem. But when you make it to easy some people will probably find a way how to use this “gap” and use this method for “they do not farmed, and only bought items for low price and sold for high price.” problem.
I dont really have a good idea to solve this, but maybe someone else have. The only idea i have is like, when you bought a item it is blocked for a trade for a specific time like 4 weeks. But this solution is not perfect.