if an economic transaction affects only buyer and seller, with a free exchange of honest information, then by and large i'd agree there should be little or no regulation.
but regulation often deals with third parties getting screwed over by someone else's economic transaction (e.g., one that causes pollution). should we abandon these innocent people? in any real system of ethics, it's a crime to steal something from a third party, and if the first and second parties agree to profit from the third party's loss, it amounts to a conspiracy. mere "regulation" is the lightest form of protection one could reasonably think appropriate here.
other regulation deals with fraud. no third party here, just a company claiming their product does one thing, but in truth it doesn't, or it has hidden costs or hidden risks. one could argue "let the buyer beware", but you know, we tried that and people died. lots of people died. society has decided that consumers aren't lab rats, and shouldn't be risking their lives when they buy a toy because some unethical boss decided to save a few bucks by using toxic paint.
the reality is that consumers without such regulations, buyers pay a very, very heavy price before the market "gets it right" and stops using ingredients that kill or otherwise harm.
the fundamental principle here is that economic transactions should *increase* the economic well-being of society. whenever it *decreases* it, that's a problem, and regulation can and should stop in.