I feel like it is hard to say what the impact would be without trying it...Wurm is very different from almost every other game.
A large part of trading isn't really 'community interaction', it's just 'how much? when? where? ok.'
But then you get things like classified/merchant ads that provide something much more than any other game provides in the way of trading. Players can build up a reputation and get return buyers/sellers by providing a great service etc...It would be a huge loss to Wurm to lose that part of our trading.
There are a lot of different options for 'improved trading'.
Whether it's the option to mail bulk goods for a small fee or list bulk goods on a merchant.
Then there is the WoW/Ebay style auctionhouse that can be a central place on each server, placed like a merchant/trader at each deed or just an interface you can open anywhere.
Even faster boats and easier filling/emptying of containers can fall into 'improved trading' as a quality of life update.
Then you have to consider the benefits and negatives each system would bring.
A WoW style Auction House would benefit buyers greatly. Faster purchase times and delivery, easier to find what they want and cheaper pricing due to increased competition. They would lose some interaction with sellers though.
As for the sellers, it would undoubtedly increase sale volume and ease of sale. On the flip side however, prices would certainly fall, as that would become the only differentiation between products listed as buyers don't care about distance from seller or seller reputation etc. Every item would be available in one place. (unless you split it per server)
That isn't even mentioning the amount of Dev time it would take to create such a system.
It would also make merchants just a system for zero-decay storage, unless the AH worked through speaking to a merchant.
Some days I wish we had an Auction House in-game...other days I'm glad we don't.
I think 99% of players can agree that a merchant/mail ledger would be a good step forward though, especially as it doesn't have any negatives as far as I can tell.