This is total bullshit. Bitcoin is a worldwide distributed effort, there are at best estimate 20-30,000 miners/cards floating about mining right now. The United States alone is made up of about 20-30million gamers. Dropping 20,000 second hand gpus across the nation is not going to be some huge glut that gums up the workings of the entire second hand market.
It's weird how misinformation seems to catch fire so much more easily than real information. That's why the OP hears "oh 6990s! awesome for mining!" when they are in fact one of the worst mining investments.
5970s are far superior to 6990s for bitcoin mining, though their resale value will be likely a bit lower.