Hm, I could steal gold and then 1 item, depending on the item lvl. You see, you have to take care of 2 things:
Level of merchant + level of item.
At pickpocket lvl 2 I could steal gold + 2 items of a low level. I needed lvl 2 to get my hands on splitting arrows (lvl 1 didn't work).
If you mean BF => you can steal up to your max lvl per act. So, if you change acts, you can steal again. I wasn't greedy, just looked into the trade window and decided if/what I wanted to steal.
Sadly, the pickpocket fun was lowered due to patch versions (Larians have no heart for thieves <sniff> )
Let's say you find something very very useful for yourself and are at low level. => max out your pickpocket lvl - loot and then unlearn this skill. But I'd only do this for very precious stuff - depending on your char level, this can be quite expensive
Oh, and if you're playing 1.44 => holy water can't be stolen anymore <img src="/ubbthreads/images/graemlins/cry.gif" alt="" />