How exactly does this work? Sometimes I sell stuff and I get exp. Sometimes I don't.

Is it determined by amount sold? Level of item? Shopkeeper sold to?

Commerce XP isn't much more complicated than the helpfile reads. The amount of money being exchanged is a factor, and there are mechanisms in place to prevent characters from building up commerce XP too quickly (i.e., we don't want you maxing it out in one session because someone gave you a couple hundred gold to burn through).

A couple things worth pointing out I suppose are that many NPC quests these days give commerce XP rewards (you'll see an echo) and interactions with NPC healers count as well.

Hope that helps.

Can you get healing at any healer NPC or just certain ones?

I always wondered if it was by design that some random drow priestess healer would heal me while I am slaughtering the NPCs right next to her.

Area authors have some options with regards to flagging NPC healers such that they won't heal certain characters. For example, the healer in Voralian City will not heal evils.

Most of the healers in Thera are pretty indiscriminate and will heal anyone whose money is good, regardless of who they are or who they are slaughtering.

Maybe this is why I'm confused. I sold a bunch of stuff and then logged out and got 0 commerce exp. I then logged in later and did no selling (can't recall if I bought anything but if so was just food) and received comm exp when I logged out.

I don't have the logs but maybe this needs bug boarding?