Trading merits for 'something' is against rules and will get you into trouble. Friends giving you merit or you giving merits to friends is alright, you can give merit to anyone as much as you want even if the post is completely shit, no one is going to stop you or take action against you. Although if you are taking merits from your alts that will get you into trouble.
AFAIK it wasnt directly against the rules, and almost completely untraceable and unenforceable, just have a look in the reputation board, youll see.