I don't really want to see this as a thing that happens.
Sure, it'll make SOME things easier when it comes to trading, but the bad parts seem to completely outweigh the good.
The economy is terrible as it is. We don't need to add in being able to trade rare adults and make it a hundred times worse. I know that the economy is fluid, which is good, but being able to trade adults will put the economy in such a perpetual state of motion that it'll be impossible for anyone to trade, especially for the people without rare dragons to trade.
I can also see the people supporting this idea complaining later on because they traded away a rare dragon that they wanted to keep later on down the road. Which would be /really/ annoying.
Also harassment, uh yeah, it'll increase astronomically. Imagine how many pms a cb tinsel or shimmer owner will get from people wanting to trade for their prize...it'll become near impossible (or at least take most of a day) to block all of those people. I have no idea how you think that harassment won't be a problem.
Anddd would you really want the game to become /that/ easy? Being able to trade adults gets rid of the challenge associated with breeding, catching and raising eggs. You'd better believe a lot of people would stop raising eggs completely if this were implemented. If anyone plays WoW, I'd equate trading adults to the level 90 boost they just released, it bypasses the core function of the game and discourages people from doing what the game is all about just because they're too lazy to raise dragons or just want everything right now. I like the challenge that DC represents.
Yeah, no. Trading adult dragons WILL break the game and I am relieved that DC is one of the few games out there that doesn't let you trade adults. It's a nice change of pace.
Edit: So many people would QUIT the game over this, making it less playable for everyone left behind. People drift away when they get bored, can you imagine how many people would leave if the game becomes so incredibly easy that it doesn't capture their interest anymore? And believe me, it would become VERY, mind numbingly easy.