What is the story behind Black Friday?

The number one trending topic in most parts of the world yesterday, including Nigeria, was Black Friday, the day after Thanksgiving where stores open early and slash prices of goods to kick off the holiday shopping season. But a group condemning the participation of blacks in the sales day say it stemmed from slavery. Really? Below is what they wrote 

"This was the day after Thanksgiving when slave traders would sell slaves for a discount to assist plantation owners with more helpers for the upcoming winter (for cutting and stacking fire wood, winter proofing etc.), hence the name"

Culled