On Amazon, textbooks, some children's publishers, collectible books, and specific titles subject to rights complaints are common restrictions. The block appears at listing time, which is why scouting apps show a gated icon when you scan.
Gating varies by account, so a title one seller can list may be blocked for another. Approval sometimes requires invoices from an authorized distributor — something a library book sale receipt cannot satisfy.
The practical effect at a sale is that a high-value scan result is meaningless if you cannot list the book. Always check the restriction indicator before adding a book to your buy pile, especially in textbook and children's sections.