Tallima: That said, running to town isn't terribly fun, either. So you may want to try an exploit if you want tons and tons of money.
F4LL0UT: Who said anything about running? I'm using the Magic Door. I don't know what the chances are of obtaining the corresponding book but you can always use scrolls. Teleporting to town and back isn't much of a hassle, plus the frequent visits make sure the NPCs approach you with sidequests as soon as possible and you may catch a lot of good deals at the merchants.
Yes. I do use magic door, too. A lot of people hate going back to sell stuff. I get the thrill of raising my bank account, so I don't mind it at all. But lots of folks are used to auto-sell systems now.
It would have been great if in single player you could send your companion away for a minute or two and they'd sell everything and then come back. It would make a nice high-challenge moment and you wouldn't have to go back to town.
Nonetheless, it's not much of a problem. Magic Door scrolls and books abound. And by selling everything you grab, you end up with more than enough money to get what you need. But not everything you want.