As long as the local rule isn't absurd then usually they are enforced if someone reports the infringing post, while also quoting the local rule. However, as suggested by others its down to the individual moderator that is handling the report on what they consider to be a reasonable local rule. Although, honestly we don't get too many local rule reports.
By the way, generally, I am not in favor of creating a self-moderated topic. Because it can be abused to delete the posts that OP doesn't like.
This is one of the ways of remaining a little less censored. Local rules seem to be put in place by the majority of users to censor someone. However, its not the full blown self moderated way of censoring. At least its out in the public. I feel like local rules could definitely be applicable in auction threads, but elsewhere I probably wouldn't use them personally.