It is a rather obscure passage, and I suspect it was not written by a native speaker of English.
"low common denominators." I think the author is referring to the fact that when one seller reduces the quality of his merchandise, other sellers tend to do likewise.
"promotion rackets." These are attempts to sell products by means of misleading promises of benefits, when in fact a buyer is unlikely to receive any benefit. Entries in a raffle are a common form of promotion racket. Buy this and you could win an all expense paid vacation for two to whatever place.
"We know too much about." I think the author means something like "we are all too well aware of", because we have all experienced these things frequently.
And lastly the author goes on to bemoan the fact that these evils of marketing affect everything from cheap disposable everyday objects, "ten cent store articles", all the way up to expensive consumer durables like cars, and even the style of buildings.
Student or Learner