Purely from a crafter/trader point of view;
1. Being able to designate items your vendor(s) will buy from other players for the value you give, with a max/min amount that vendor will buy. No need of having to wait for players to come online for a trade or spend days/weeks trying to meet up.
2. Remove vendor fees. Useless and pointless system. Make the vendor deeds way more expensive instead, so much easier.
3. Expandable backpack (as in third dawn), allowing you to stall out all your items for sale in one view. And not having to browse multiple (empty) backpacks/containers just to see what's available. Allow seller to customise the view (without going into detail, shouldn't be hard to imagine what i mean).
All in all, make it less such a chore to do simple things, bring this old relic of the past of doing business more up to date and visually appealing by letting the owner add customization, and make it more fun to run a business where the owner can concentrate on producing/acquiring goods instead.
Or to consolidate these 3 points into 1;
4. Remove vendors altogether. Add a menu choice when single clicking a house sign (public or private), such as "enter shop", which brings up a new resizeable screen showing everything the owner has for sale, including a buying section for the owner. A display case you can place on a house plot and double click with these options in mind would have the same effect.
But to be fair, UO is old and is just way beyond trying to get it up to date. It's more about trying to fit in new things within the capabilities of an outdated environment as far as I'm concerned.