In common commercial english, i would read that as “this merchant will offer to trade any of the books for an amoumt of currency equal to half the book’s cover price plus $1.”
Such vagueness also suggests sufficient informality that the merchant may either accept seperate offers or veto the general rule on a case-by-case basis.
I think you’re underestimating how deep the pay gap goes.
“women’s work” jobs are so consistently paid less that mere career choice is a huge part of the pay gap.