Apparently saving and reloading will reset the inventory. I haven't done it personally, but people are doing it to reroll for the skill books they need.
Saving and reloading the game does not reset vendors.
What we mean is that when
you have reached a state that triggers the inventory being rerolled next time you talk to the merchant, then we save before talking to him, talk - and if we don't like what we see, we load the save we just made and talk to him again... and continue doing that load/talk until we get what we are looking for or conclude that this vendor at this particular point of progress is not going to have the skill we are looking for regardless of how many times we force it to generate an inventory for us.
For the easiest example of how this works, save immediately before talking to a merchant for the first time and do the load/talk dance from there.