If you see somebody selling merit: make a post in the reputation section and/or report the seller. Both seller and buyer will probably receive -ve from DT and get their accounts ruined (including whatever they invested in buying merit).
Selling merit is prohibited, however this doesn't mean it isn't going to happen, there are a limited number of mods and and admins... They can't monitor each and every thread, let alone monitor off-forum sales. That's why it's important we report these cheaters
Selling and buying of merit is prohibited and the act can be reduced to certain extent, but what about people that make multiple accounts? They earn merit in one and send it to their other accounts. Its like a chain as merit receiver also receives half of the number as Smerits. There is no system to check these type of frauds. People will continue to make multiple accounts unless phone numbers are linked to people bitcointalk account and 2 factor authentication is made mandatory. Another alternative to stop this is by making KYC mandatory, but people would hesitate to reveal their private information to a forum which is not operated by any single person or authority.