What about only allowing alts to trade with other alts (and not with mains) ?
Can't you switch your main to an alt now? That would leave the discussion back at square 1.
If you want an alt so that you can have an alternate character, what is the issue with treating that character as a separate character? Everyone knew exactly what they were getting into when they made an alt, i don't understand why it's an outrage that one cannot now do what could never be done before.
The advantages to my alt are having multiple playstyles available, the ability to custom pick your character for the map or opposition (cav, ranged, shielder), and a greatly decreased chance of burnout. They pay for this by splitting their time and goods across multiple accounts
The advantages to having a single main are increased progression and easier acquisition of looms. The downside is that a single character player is locked into his choice and cannot adapt to changing map conditions but must make do with his build constantly.
Why then should alts get another advantage? Seems pretty balanced as it is. If you want to have a main you get the benefits of having a focused character progression. If you like alts you get the benefits of having lots of choice.
Remove the barrier between alts and people who have alts suddenly get all the benefits of single character players with none of the downsides. Suddenly it becomes stupid to not have had an alt, rather than a balanced choice between two equally viable options.