To circle back to the original article, in addition to doo-wop, there are (at least in the midwest U.S., and likely elsewhere) several genres which are basically impossible to sell. Part of this stems from the economic crash in 2008.
Most general rock records (60s/70s), not Beatles or specific genres like psych, garage or rockabilly plummeted when the economy crashed. These records have been bringing 25%-50% or less of what they brought 5 years ago, and this has been further been expanded upon with the latest USPS postage rate increase in January.
Also, 50s rock in my part of the world is impossible to sell. By this, I mean Elvis, Ricky Nelson and that sort of stuff. I don't even touch it when offered it.
I get calls from people wanting to sell me big band, classical and 50s pop, to which I tell them there's no market and to donate it to a charity. As one woman said to me on the phone: "But they're such wonderful records." I said that may be true, but nobody wants them, and those that have them, are dying off and wanting to get rid of them.