Since I've been trying to make a list of who can purchase what skill, I am pretty sure by now that what gets offered seems to vary by:
Person selling
when in the game I ask for it
the level of the skill
maybe something else?
Often different sellers of the same skill at the same level, or at a different level, offer the skill to different players, or even if I go back to a seller whom I bought everything he was offering he would now be offering to a player he didn't offer it before. (It's even possible that I have been sold the same skill to the same person more than once, but that could be my bad memory.)
What is the truth here? Is it determined by the seller, or is there some stat that determines when a certain skill can be offered to a particular character. Or is it at some point in the game now that skill is available for some character. Of course all along some characters could never earn some skills, but this is beyond that.
Thanks.