One way to do it is to overwrite the trader treasure tables (in the stats editor) and add your books to them.
Thanks for the reply, I read your reply in a similar post. But how do I do that? The guides teach me how to create a custom vendor, add my skillbook to his treasure table and then place the vendor in a custom level, but they still don't help me to:
1: Load Origins levels, not custom levels.
2: Locate relevant vendors. (e.g. I want Kalias in Fort Joy to sell my skill book - where is he and what is his treasure table?).
Once I know where the vendors are, it's a simple matter to look up which treasure table they're using, then add my skillbook to it.