I don't want to be disingenuous, so I'm going to say ahead of time that this request partially comes from a place of selfishness and I'll explain why in my reasoning.
In the Japanese version of pso2 players are able to sell, buy and trade for higher-star gear than we are. The Japanese version of the game allows this because they understand that getting 3 different pieces can be very difficult, and despite the difficulty, getting 3 compatible (not necessarily a set bonus) units is a vital part of the "endgame" regardless of what point in the meta we're at.
As for my personal desire for this: I'm tired of getting nothing but Ofzende arms, and Cliffard arms. I've gotten so many of them and it bugs me to no end that I can't buy or trade for the pieces I lack, or sell the extras to make a bit of profit. I honestly thought this would become a thing once 12* units and 14* weapons became a thing, and it's extremely disappointing because it's another route that NA could have taken to learn from the downfalls of the japanese version.
For example: Japan released SG scratch, everyone hated it so they intorduced more, and better ways of obtaining SG. And instead of giving us the scratch along with the solution to the problem that initially arose from the problem it created, we followed japan's erroneous footsteps and got the problem first with no solution. And now this issue, while not as big is present none the less. NA would have been better for allowing these trades when stronger weapons and units started appearing, but instead we once again repeated the mistake that they made in the japanese version instead of giving us the solution along with it.
So TL;DR: My suggestion is giving us the ability to trade higher star equipment like the japanese version does.