But you haven't solved any of the many issues player to player trading causes, that is detrimental to the game itself. You can keep on brining up specific cases where everything is fine, and trading make sense, but that is just hiding the ugly things...
How will you deal with gold sellers?
The new incentive for bots in the game to collect material?
The new influx of accounts that is bought with stolen credit cards?
Spam in chat? ban the account? oh yea, that account bought with a stolen credit card....
This is stuff you can go to just about any other MMO game out there and watch, if they allow player to player trade.... So this will create more strain on the server infrastructure, it will cause more support cases, as more people will report the spam bots in chat. Not many bots collecting material will be reported, as these will most likely run in solo only. Then we have the entire mess with accounts bought with stolen credit cards, more resources diverted back to manage that mess.
Just so that a few case where tradiong makes perfect sense to be available. And this is not a new thing, and we are still waiting for a good design to manage all those negatives... and what have they came up with? auction houses, where you are not able to trade directly player-player, and we all know that auction houses have their own fair share of issues.
One the biggest issues that these brings is huge inflation, into a game mechanics that does not have an inflation. Once again, bots are going to be deployed to spot all the valuable stuff sold for cheap, and then put out again for much higher price.
...solution to this is fixed prices, auction house have set max limit on how much you can charge, and if the limit is set to low, very few will sell this, setting it to high, there will be ALOT of complaints from players that do not have the fund to buy the stuff they need.
So I do not care how useful player-player trading can be in certain situations, all of the rest of the negative stuff is going to ruin this so we are better of without player-player trading.