Word and phrase origin questions are interesting for a number of reasons, including their inherent complexity. "Cheaper by the dozen" demonstrates how hard it is to construct a satisfactory origin story. First, we have the mystery of why dozen itself caught on instead of some term for a group of ten (say, dixet). Then we need to determine whether "by the dozen" was a set phrase before "cheaper by the dozen" became one—because if so, the addition of cheaper doesn't seem an especially earth-shaking development. Then we need to figure out when "cheaper by the dozen" began to appear in written English—and in what context.
Having done all that, we have still dealt only with the written record. What about oral traditions concerning the phrase's origin? This is where the question and one of the answers posted on the main site come into sharper focus. They seem to argue for an origin story for "cheaper by the dozen" rooted in the U.S. slave trade of the 1800s that is based primarily on oral tradition. That is, citations to explicit written evidence go back no farther than Dick Gregory in the 1960s or 1970s, although some later writers echo and amplify his analysis.
I tend to be skeptical of folk etymologies—not because they are inherently devoid of any truth, but because they tend to be too neat and convenient in reducing a messy history of usage to one simple point of origin. Nevertheless, we are stuck with the reality that most slang and many idioms originated in spoken English and only later were assimilated into written English. It is thus problematic to depend on written English for definitive proof of a phrase's origin, especially if the phrase is obscene, profane, or otherwise offensive, or if its origin goes to an unsavory past that polite company would not wish to acknowledge.
So in weighing the merits of an oral-tradition-based claim regarding a phrase's origin, you have to balance the possibility that written acknowledgment of the origin may have been suppressed for various reasons against unsentimental consideration of what actually appears in the written record. An origin story that claims usage going back to 1840 in a particular context is not refuted by an absence of any mention in print of that usage in the relevant context until 1966—but it is subject to reasonable doubt for that reason.
A case in point is the phrase "cheap dozen[s]," which site participants mentioned several times in the course of the "cheaper by the dozen" discussion as a peripheral term in the context of nineteenth-century slave auctions. I searched Google Books and Elephind for instances of this phrase in the context of selling enslaved human beings in a group of 12 at a discount and couldn't find any reference to such sales from earlier than 1974—in Middleton Harris, The Black Book, cited in Lisa Green, African American English: A Linguistic Introduction (2002):
On some accounts, the term dozens was used to refer to the ill or old slaves who were sold in groups of twelve (Harris 1974).
The earliest explicit use of "cheap dozen" that I’ve been able to find in this context is from Mona Lisa Saloy, Still Laughing to Keep from Crying (1990), quoted in Uncle John's New & Improved Funniest Ever (2018):
The Dozens has its origins in the slave trade of New Orleans, where deformed slaves—punished with dismemberment for disobedience—were grouped in lots of a "cheap dozen" for sale to slave owners. For a Black to be sold as part of the "dozens" was the lowest blow possible.
So again we have a term with a claimed origin that is more than a century older than the first (known to me) print instance of it.
I don't know how professional historians appraise the legitimacy of factual claims rooted in oral history; but in the context of word and phrase origins, I think we are on far solider grounds when we focus on identifying support for such claims in the written record—the older the better, in the case of actual usage.
For this reason, I was very pleased when the poster who had provided a largely oral-tradition-based answer to the question about "cheaper by the dozen" followed up a day or two later with a question about the earliest occurrence in written English of the phrase "white race[s]" in the context of racial theory. This is an area where EL&U participants can offer potentially useful information: not in propounding some claimed oral tradition that ties race-focused thought in English to the origin story in Heimskringla, but in identifying when and where early instances of the phrase in question appear in the written record.