There's a lot of grey areas to multiboxing, but it boils down to this: you shall not receive honor or experience on both accounts at the same time, but professions, banking, etc are all in the clear.
Viper has created a thread that does clarifies multiboxing a bit here:
https://forum.nostalrius.org/viewtopic.php?f=2&t=2812In the Terms of Use, he clarified that banking and professions is allowed behavior:
https://forum.nostalrius.org/viewtopic.php?f=2&t=8Clarification on multiboxing
Multiboxing refers to playing multiple separate characters simultaneously. This can either be achieved by using multiple separate machines to run the game or by running multiple separate instances of the game. It is against the rules to /follow level characters, run dungeons or raids with multiple characters, PvP with multiple characters, etc. There are only a few situations where multiboxing is allowed in Nostalrius, e.g.:
Trading items between accounts.
Professions, as long as no farming is involved;
i.e. You can have two characters ingame simultaneously for crafting, enchanting, or trading purposes.
It should be safe to assume sitting at an auction house on a character is indeed banking as commonly used in the US which would be withdrawals (gold/items), transactions (gold <> items, items <> items), and deposits (gold/items).