A bank alt can have a small inventory to maintain just fine (bare min runes required for portals). Most of the storage is in the mail system. For trips between mail and AH, emptying bags into bank first is easy. For additional storage can make a few lvl1 bank alts.

"just a alt...not a bank alt"
An alt, permanently parked near the bank and AH...that you mail things to in order to sell, and use to collect things you've won from AH.
Could you lower your certainty that it's not a bak alt from 100% down to 99% at least?

Can we agree that it's something similar to a bank alt, if not actually a bank alt in name, and that it can and does perform every single duty of a bank alt?

No?
I guess it's not a bank alt then, but whatever it is...it does everything a bank alt does and more.