selling of miles (or award tickets) is legal, but in most cases against airline rules (small print).

For the miles&more program (Frequent Flier program of Lufthansa, swiss, austrian, polish) there is a (indirect) way to do it not only legally but also without hurting any airline (or other small print) rules.

This has been done openly many thousand times (and is still done now) and several threads about this are found in the swiss airline forum (a subforum of the miles&more forum).

One of the latest FlyerTalk discussions and tips how to do it can be found here.

I myself have traded several hundred of millions of Coop Superpoints and converted them (regular convertion rate is 2 Superpoints = 1 mile, but from time to time there are promotions like 3 Superpoints = 2 miles, or in the early days, some 4 years ago the promotion conversion was even 1:1 - and from that early promotion days I myself still have over 3 Million miles&more miles in my own account) into miles of different miles&more accounts (including m&m accounts of at least 30 different FlyerTalkers).

Coop Superpoints can be easily and legally (and supported by the Coop Superpoint homepage) traded online (no costs involved), and the conversion into any miles&more account can also be done online on the Coop homepage. The trading of these points and the conversion into any miles&more account are not only within any Coop small print rules, but are even supportedd by Coop online. EVeryone can earn Coop Superoints when shopping in swiss Coop (and partner) outlets. And Superpoints in huge quantities are traded daily on swiss auction platforms (leader of such an auction platform in Switzerland is Ricardo, only second and by far not as competitiv as Ricardo is ebay here)

All of the above has been openly shown and discussed in threads and posts on FlyerTalk (always in the swiss airline forum, now the swiss airline forum is a subforum of miles&more).