I teach at a community college, and my experience with textbook reps is that they're very friendly and persistent, but ultimately all they've really done is to give me a copy of their book and try to convince me to adopt it. There are certainly many unethical things about textbook publishing, but as far as I could see, they were decisions made by executives in New York (e.g., bringing out new editions to kill off the used market, or shrinkwrapping the book with useless trinkets so that it couldn't be returned). In online discussions, I've often heard people who seem to be students make claims that professors were "bribed" to use a particular publisher's book. I never believed these claims, and when I asked these people to supply evidence, they never had any.
But recently I was talking to a colleague in another department, and he told me that his department would never change their book for a particular course, because his colleagues got ethically questionable inducements from the publisher. When I asked him what they were, he said that the publisher would, e.g., repeatedly invite faculty who were using the book to "meet-the-author" events in Florida. He said that these faculty had grown to expect this as a perk that they were regularly offered as an unspoken quid pro quo for continuing to use the book. I didn't ask him for hard evidence of his claim, and I don't know what form of evidence he would have available.
Is there really any hard evidence that publishers do this sort of thing, or is this an urban folktale?