It means they've slashed prices so low they don't dare advertise them.

It dates back to a incident that occurred some years ago when a dealer, in an effort to pass along savings to his customers, advertised an A-1 Special Parker for $7.50. The resulting stampede caused several deaths, and the dealer was unable to satisfy all of his customers who wanted to buy the gun. That lack of satisfaction, and to a lesser extent the deaths, created the POR.