If you can get your alt into the same FC as your main, and you have access to items through the company box, then you can transfer items through the company box. Obviously not the highest end solution, but that's the current way you can sell transfer items between alts, though it requires the same server.

Seeing as they won't even let you check your moogle mail on the same character when you world visit, alts on other servers and DCs won't ever be interacting at all with trading or anything.

As far as getting to skip your alt up to max level really quick and all that jazz, seems to defeat the purpose of an alt to me. I have 5 at Shadowbringers, with 3 at Savage Raiding(though I'm hiatus this tier).

There's a large portion of the playerbase that are altphobic or don't understand alts in general. This ranges from the roleplaying community looking down on RP alts to conventional players looking down on raiding alts. The feeling is the same though, you on your alt are wasting their main's time/effort. A real puzzler for me, since I mean, even if I'm playing an alt I'm still contributing just the same as I could on main, still spending my time too etc. I don't think they're ever going to put in measures to make having alts more desirable.

I would love for achievements to be shared though, but there's probably a programming reason for why they haven't done that, in addition to the idea that some players are teens or even adults sharing an account playing different characters.