In North Carolina, slaves were considered to be personal property, while in Virginia, slaves were considered to be real estate. That said, you will find bills of sale for slaves in NC in the deed books, but you'll also find some other bills of sale in those records, too.
In the NC counties where I've done research, the marriages of people of color pre-emancipation are found with the regular marriage registers. It's not until after the Civil War that counties started with having a separate book for colored marriages, and then that ended for most counties before the 1880s, I think - definitely by the 1890s, at least in Washington County. How long the separate marriage registers were maintained might have been something that varied from county to county.
The link that you have seems to be to pages in that marriage book that are referencing the records of cohabitation, as they all seem to be dated in 1866. Most of the time, you'll find the cohabitation records in a separate book from the marriage register. It's interesting that it's worded as freedman or freed persons, as the cohabitation register was for former slaves to have a chance to have their 'unofficial' marriages legally recognized, and by 1866, everyone was freed. Washington County's looks like this:http://www.ncgenweb.us/washington/mr/WFREE026.HTM
Referring back to a previous post about slave marriages being listed before the 1860s, those were church records, not county records. Churches could record the marriages of their parishioners any way they wanted to, even if the law technically forbade the marriage of slaves. And that could be why the church records refer to the couples as 'servants' instead of slaves. People have been finding ways to get around the government laws for a long, long time.
I'd say that if a couple appears in the marriage register or marriage bonds prior to emancipation, and are noted to be colored, then you know that they were free people of color. And if they're a head of household in any census prior to 1850, then they're free people of color. What you don't know for certain is were they Native American, African American, or mixed race. Court records could tell you those answers, especially for men, as once they reached adulthood, they needed to have papers on them to prove to patrollers that they weren't slaves. They'd appear in court, usually with at least one white person to testify to their character, and a physical description is often given. And of course, we don't have those records for Washington County. I've not yet gotten into the Court of Pleas and Quarter Sessions records (which is where you'll find this) for Beaufort County yet, so I don't know how complete their records might be). And sometimes when a child was apprenticed, you might find where one of the judges or the clerk himself has described the child as a free black child instead of a free colored child. Not that that's all that accurate, as you've seen from the census records that different people could see skin color quite differently from one another.
If you could find what church James Magee attended, you might find something there about David. He would have attended church with the family, and if he was baptized in that church, then there might be information on his parents in the church register/records. I just checked the cemetery book for Plymouth, and James Magee is not listed (I was hoping to find him in a church cemetery). I know that Grace Episcopal Church has their records on microfilm at the state archives and most likely also with the Family History Library. I don't know how many other churches in Plymouth have their records on microfilm.