I think rhe voting age should be the lower of the minimum age to labor or the age of potential conscription less the age of the longest-term official whoss job includes sending people to war.
In the USA, that would put the voting age all the way down to 12. And having both been 12 myself once and having close family who were recently 12, I’m entirely OK with that.
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.