Does purchasing books from vendors at the exhibition before its opening—after they have been displayed and priced for foreigners—count as receiving the caravan (talaqqi al-rukban)?

It is permissible for vendors to purchase books from bookstores before the opening of the book fair, because this does not fall under the prohibition of "intercepting the riders" (talaqqi al-rukban), which refers to buying goods from outside the city before they reach the market at a lower price, thus causing injustice to the seller due to his ignorance of the price. As for bookstores, they know the price and there is no harm to them.

However, if the books are subsidized or rare, and merchants exploit the public's inability to enter and buy these books, then sell them at exorbitant prices, this harms the public and undermines the purpose of subsidizing books to reach people at a low price. This may also entail violating the condition set by the book's owner. The Prophet, peace and blessings be upon him, prohibited causing harm and reciprocating harm.