I would love to see this, but I feel the reason they don't do it is it would result in higher age players farming artifacts for their alliance, overpowering the lower ages...in addition to the usual money grubbing claims.
To combat that, make it so artifacts traded are reverted back to base level, which lower ages will have considerable effort unlocking. Not only that, but charge a crown fee for each trade, 50/100 crowns for MP/War artifacts and 150/300 for legendary MP/War. The crown fee would be partly to reduce flooding the lower ages and also so BHG can take their pound of flesh