Banker, mule, or Auction House Alt, the idea is the same. The [tag-tec]World of Warcraft[/tag-tec] has made it easy for us to make many different characters so lets take advantage of this.
The Banker/Mule/AH Alt is a character who stores, buys, and sells stuff for your toons. All of them.
Gather up some collectibles, hit the nearest mailbox, and send them all to your banker. Log into the banker at some point and post your auctions, collect your successful sales, buy stuff and ship to the appropriate other character, etc.
The major share of all your gold will stay with the banker. Partly for convenience, partly to stifle impulse spending if your main is careless with her gold.
Roll up a toon and run to the nearest auction house, at level 1. Level it to 5 if you want to make use of skills such as disenchanting, tailoring, etc.
I like Stormwind on the Ally side, but Ironforge and The Exodar are fine. Darnassus’ AH and mailbox are too far apart and I’m lazy. 😉
Horde side I like the west AH in Slivermoon since the mailbox is about 5′ from the AH, but go where ever works for you.
Never see the Ah again
Rather, your mains won’t. Anytime they get stuff, they send it to the mule. Anytime they need stuff they send a purchase order to the mule, who then buys it, packages it, and sends it off.
One trap here is forgetting exactly what gear your main has. It might be worthwhile to send some cash from your banker to your main and have your main buy it directly. She’ll have to do that with Enchants anyway, so what the heck.
Need some extra short term storage space? You main sends the stuff that needs to be stored to the banker, who then returns it. It’ll sit in the mail box for up to 30 days and you can pick it up as needed. The mail system will hold far more stuff than your bank and it’s more convenient, it just won’t hold it forever.
About that level 5…
If you level your banker to 5 you can pick up the enchanting skill and level it to 75 if you like. You can then disenchant all those worthless low level green items you pick up.
If you grab tailoring then you can add a minor health enchant to the low level chest items you can make. This might allow you to sell those items for a bit more on the AH.
You can also make up some of the basic green items, disenchant them, and sell the parts.
So save yourself some time and effort in the future and roll that banker now.