Though widely used, it's technically improper. In fact, English lacks a generic singular personal pronoun. The best way to get around this might be to make the subject plural:

If the moderators think this post is more appropriate for main they can move it there.

EDIT - removed irrelevant links and snarky comment about downvoters, plus:

Researched further, my judgement was hasty: Although some usage writers condemn the use of the "singular they" when the gender is unknown or unimportant, this is often used, both in speech and in writing (e.g. "If a customer requires help, they should contact..."). In fact, a consistent pattern of usage can be traced at least as far back as Shakespeare, and possibly even back to Middle English. It avoids awkward constructions such as he or she. This usage is authorised and preferred by the Australian Government Manual of Style for official usage in government documents. See Singular they. The use of the "singular they" can often be avoided by thinking ahead and rephrasing the whole sentence (e.g. "For assistance, customers should contact...").

From this Wikipedia article.

I am neither the grammar police nor the editor of an English language style guide.