We prefer to not make rules for problems that don't really exist...it helps keep things simpler for everyone. To date, instances of people selling things in sigs have been few and far between, and we haven't received reports of problems stemming from the ones that have appeared.
Deals in the Marketplace forum, however, have long been a source of problems, which is why we tightened up the requirements. Things have been orders of magnitude better since we made those changes.
But as SilentPanda says, if sale/auction listings in signatures become a problem, we'll address it.