I don't think that is true. I know of intra-family sales where that definitely didn't happen and a quick search found nothing. There may be some internal rules

I don't think that is true. I know of intra-family sales where that definitely didn't happen and a quick search found nothing. There may be some internal rules among Realtors (who own the MLS- it isn't a public service) that require that, but a law declaring that would be dictating paying for a given private organizations product, which sounds problematic.