To start my first real post here with something obvious:
I think many of us here are in the same situation. We are planning our first 4th Edition campaign, reading every tidbit of info and analyzing how it will work with how we like to DM and the game we have planned out.
My gut reaction was negative to things like buying and selling magic items and automatic identifying what the items are for. My gut reaction to disenchanting/reenchanting was more of "Hmm that sounds like something I can work with..." And then it was time for my process. Why do they want things changed? Why did I react the way I did? Does this work with my campaign as it is? Can I make small changes in either the campaign or the rules to make it work? How would my players want it to work?
The way I feel right now I think it is the right route to have generally permissive standard rules in the books, and I have no reason to doubt the game balance in the 20% resale rule. I suspect that my gut reaction has to do with being an oldtimer gamer, used to playing in environments where we avoided all sorts of houserules (as much of our game was playtesting for conventions), and skeptic to change until have tried it. I do like the idea of making the game easy to manage for DMs and fun for the players, avoiding unnecessary restrictions and busywork for both parties. And these changes are clearly in that spirit.
On the other hand, with this complete information, I don't think I will play exactly the way rules are written in my campaign. My campaign world is simply not civilized and affluent enough to support magic item trading in general, that would be restricted to very few occasions during the story. Also, both with my story and my players' preferences in mind, magic items they find should be a little bit more mysterious.
So, my preliminary houserulings are these:

Merchants very seldom have magic items for sale, and it is even less known for them to want to risk buying them from adventurers. (Not much chance of them finding a buyer in my little world.)

On the other hand disenchant/reenchant might be given a slight boost, in that I might let it be a bit more powerful, but limit it so that the residue of the magic from the old item will spill over into the new one. In other words, if you take the power from a fire oriented item it can only give fire oriented power to another item. (This fits in real well with how my world works, and should make the game more fantasy and less numbers - I think.)

As for identifying items I will let them automatically identify everything that is up to their own level of power. The more straightforward powerful items they will also id after a few minutes of testing, others will take arcana checks. But. In no case will I promise them that the information that they have is the whole truth. (And in some cases it really will not be.)
And how does this post fit in with the general debate in the thread? I think we should remember that they make the way they think will be the most fun for the most people. And that as a DM it is sometimes a bit scary to give up control, but players can be trusted to want to have fun too, and not want to ruin the game. On the other hand, a strong story or world concept sometimes has to come first. So I might have to change a few things around, still trying to not be more restrictive than I absolutely need to in order to make the stories work.
(Sorry about the long post. Will try to hold back in the future.

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