Despite not having much to suggest myself, I'm going to poke holes in your suggestion.
With the suggested system you are preventing low level players from selling potentially really valuable rare items at an appropriate price.
The amount of movable silver would also have to be adjusted for inflation, and couldn't be universal for every server.
Players would also need a means of knowing what their current amount of movable silver is.
Forcing players into any behavior is a no-no. Some people don't like quests. Some don't like grouping. Some don't like dungeons.
There are even players who legitimately like grinding and will sit in a single mob spawn for 10+ hours straight, and will do solely this to get to cap level.
You would have to set the requirement for any such system as something that EVERY player is going to reach, but will hinder bots/gold sellers. Perhaps something like 10,000 mobs, and 10 bosses killed to unlock the auction house. This gets bots out into the open, and requires that gold sellers even put work into their accounts that will sell items, while only mildly inconveniencing players.
Though this is altogether pointless if gold sellers/buyers aren't caught and banned as no amount of work will stop them if they only have to do it once.
As far as 1:1 trading goes. Just restrict it to no silver trading.
Some people will balk at the idea, but players will find a way. If silver can't be used to buy items, then a commonly used item will be. For example, potions.
If potions aren't universally needed, then enhancement items.
The bottom line being, there IS something that can be substituted for silver.
However, I believe this all to be moot as IMC games real intent is likely just to monetize trading and tax gold sellers.
The only way that will change is if there is a huge communal outcry over it.
Ooops, I didn't intend this to be such a large post.