There’s never enough bag space, and it is way too complicated to have more than one toon use the Auction House for selling items. So make a bank mule and use it effectively!
Many players I know use a mule (or “bank toon”), to store items, sell items, or both. It is far more efficient to send saleable items to a toon parked at the AH, one with bag space.
An additional benefit of mailing to the mule is that this makes that toon’s mailbox a “virtual bank” with – in theory – 600 slots! In practice, you can easily have 150-300 items, if you mail them in groups.
Finally, for even greater storage, have your mule set up his/her own guild to get a guild bank. Guild banks can hold 98 items per tab. With a modest investment, you can readily add 400 or more slots to your bank mule that way, depending on how much you’re willing to shell out. To set up a guild bank, go to the guild organizer, and tell people in trade that you’re looking for signatures. There are often low-level toons without a guild scrounging for gold – if you offer a generous tip for signatures, you should get enough in no time.
Once your charter is signed by enough others, turn it in, kick everyone from your guild, and go buy your guild bank tabs!