Here's how it works, basically if you want 200+ extra slots of inventory and the ability to sell things (to other PCs)... go into the adventurers guild or whatever it's called (There's one located in each city) and talk to all the NPCs near counters. Eventually you'll talk to one that asks you about retainers blah blah blah, pick your retainer (You get two), name them, and send them on their way. You can access your retainers through little retainer bells (They look like bells on top of a large brown thin pedastal.). Next step...
TO THE WARDS! There are wards located in each city, I haven't played since June, but apparently now you can make your purchases directly from teh zone screen into the ward of your city (You used to have to pick a specific ward, such as Grocer's Ward if you wanted to go buy foodstuffs, go there, and look through all the vendors for what you wanted). Basically now, it works just like an auction house, except now, you have neat little avatars(that you can dress up ^_~) that sell your things and you can sell most than any other MMO, and hold more for that matter too. Wish they would have got it right from the get go.
But I'm sure you'll ask later on "But wait! What if I just wanna send something to someone?"
Tough luck, they have yet to invent a mail system, however it's been released that they're implementing it soon.
I think that about covers it for Player Character Item Transactions, if you have anything else, feel free to ask.