When an author writes a book, it is frequently licensed to one company in the United States and another company in the U.K.
If the publishing company Bathroom Books gets permission to print and sell the books pursuant to a license from the author in the U.S., that contract will usually include a non-compete clause that prohibits Bathroom Books from selling books it prints to retail customers in the U.K. or to wholesale customer that intend to sell the books at retail in the U.K., where the permission to print and sell the books pursuant to a license from the author has been granted to WC Books (and visa versa).
The clauses are about enforcing the peace between distributors given different territories, because different publishers have better sales networks in different places.
The first sale doctrine, clarified for purposes of U.S. copyright law in Kirtsaeng v. John Wiley, makes the "not for sale in the U.S." language inapplicable under U.S. law to a consumer who purchases the book in the U.S. and then resells it used to someone in the U.S.
Whether it bars someone who purchased the books on a wholesale basis in the U.K. from reselling the books in the U.S. depends to some extent on the nature of the contract between the wholesaler and the publisher. For example, if the wholesaler is selling the books on a consignment basis for the publisher and has a right to return the unsold books without payment to the publisher and doesn't have to pay for the books sold until they are sold at retail, the first sale doctrine likewise does not apply.
It also isn't entirely obvious that the "first sale doctrine" makes it illegal to count U.S. sales towards the amount due to the U.S. publisher rather than the U.K. publisher, even if they are sold by WB Books instead of Bathroom Books. (Re the faux names of the publishers, one of the first books I every handled copyright and licensing for, a year out of law school, involved bathroom humor.)
Also, if the case ends up in a U.K. court before it reaches a U.S. court, the first sale doctrine may or may not apply, but Kirtsaeng v. John Wiley will not be binding precedent, so the applicable rule of law might be different. And, it isn't hard to write a binding forum selection clause that insures that the case would be resolved in a U.K. court rather than a U.S. court if the law in the U.K. were more favorable.