This leads to the wrong conclusion. The argument is about what decisions consumers should make, not about how chain bookstores should conduct business.

According to this principle, if chain bookstores tend to reduce the available variety of books (and indie bookstores don’t do that), then consumers should only buy from indie bookstores. That fills in the bookstore owner’s assumption and bridges the gap in the argument!

The idea of the bookselling business’ best interest doesn’t appear anywhere in the argument, so this can’t fill in the assumption between the bookstore owner’s premises and conclusion.

This isn’t triggered by the argument, because we don’t know whether or not chain bookstores deliberately force indie bookstores out of business. That would be our own arbitrary assumption.

There’s nothing in the argument that mentions consumers not having access to any indie bookstore, so this doesn’t trigger. The argument also never says that consumers should buy from chain bookstores, under any circumstances.