The notion that sellers can outnumber buyers on down days doesn’t make sense. The number of buyers must always equal the number of seller. What the newscasters should say, of course, is that prices adjusted lower because would-be buyers weren’t prepared to pay the former price.
I believe it is very difficult, if not impossible to predict the future, certainly over the long run. And, it's much less possible to make a profit from guessing the future.
Roger Gibson, the author of the must read book "Asset Allocation: Balancing Financial Risk," shows that including some portion of commodities in your portfolio of well diversified equity, bonds, and real estate produces better compound average returns--higher, risk-adjusted returns.