I seen stuff like this before, and I think I know how it’s supposed to work.
The “stranger” recruited the person who posted this to OP’s friend as one of 6 people. Each of those 6 people recruit 6 others, who send the books to the stranger (their friend’s friend), hence the 36. OP’s friend will be asked to recruit 6 people in turn, and each of those recruit 6, who then send books to OP’s friend.
I think it’s a bit dodgy that they’re being asked to buy new, but I’d be more concerned that these kind of schemes can be used to feel out who’s likely to fall for more sinister scams, as people feel they’re getting a lot back for very little.
Plus, it only takes a few levels (13 if my maths is right) before there are more (far more!) recruits needed than people on earth.
Psych too