Enslavers did not engage in eugenics projects where they would breed particular slaves and not others or selectively cull slaves. However, when they went shopping for people they did have things in mind.

If you've got a strong stomach, imagine you're in the market for a person. For convenience, I'm going to approach this as though you're buying through a slave market. That's not necessarily representative of most slave sales (many are person-to-person affairs) but the markets generate a larger footprint in the records and may capture the pure act of buying a human being better.

Depending on what you want the person for, you may have differing priorities but these are all concerns:

Age

You don't want a slave that's too old or too young, as you'll get less productivity out of them in the near term but have similar upkeep costs. A prime slave is past puberty, grown enough to work like an adult and know the basics. There's a tendency to prefer female slaves a bit younger, because their value comes in part from their "increase", which is their ability to make you new slaves through reproduction.

Health

You want a slave that looks healthy. All the parts are there and look functional. It was common to make men, and sometimes women, strip so they could be fully examined. (Women could be examined more thoroughly behind curtains or in a closed room.) Slave brokers would make an effort to clean slaves up and do things like oil their skin to make them seem more vital. A buyer could poke and prod about looking for sores, tumors, bad teeth, etc which might indicate a sickly slave. You don't want a sick one, as that person could die. We have cases of slaves who were passed off as healthy, but died of illness shortly thereafter where the buyer sues the seller for delivering faulty product.

Color

There's a tendency to prefer darker-skinned slaves for field work, though it's not as pronounced as the old nostrum that light-colored slaves work in the big house and dark on the field would have you believe. Lighter skin could be taken as a sign of race-mixing, which could produce inferior slaves in the thinking of the time. The darker a slave's skin, the more "naturally" suited they were to slavery. Depending on the situation, visibly multiracial slaves could be seen as either big flight risks (they can pass and may have inherited white intelligence) or just decidedly inferior as they're taken to be infertile, mentally-impaired hybrids.

Lighter skin is somewhat more valued in women, particularly women sold as "fancy" slaves. A fancy slave, I'm sorry, is a slave one buys to rape. There were dark-skinned fancy slaves too, but lighter ones seem to have commanded a premium. A buyer might inquire as to the precise quantum of enslaved blood, with a preference for a certain degree. This is the origin of some of the more nausea-inducing terms you see. A quadroon was one-quarter black, an octoroon one-eight. Slavery is hereditary through the mother, so it doesn't matter what the kids' blood quantum is or how they look.

Skills

Many slaves are bought just as field hands or domestic help and don't necessarily need advanced trade skills, but having them increased a slave's value. A slave who is a good cook or knows carpentry can add more value to an operation than one that doesn't. When Jefferson reluctantly freed his enslaved chef, he required that the chef train a replacement before manumission. Some enslavers would arrange for their slaves to learn trades in order to boost their value with an eye to future resale as well as to increase their usefulness while they were around.

Behavior

This is a huge judgment call on the buyer's part. You want a slave that looks "likely". That includes all the above, but also an apparently obedient nature. Slaves who look like they've got a propensity for, or have proven records of, running or other resistance are much less desirable. The right overseer or slave breaker might beat that out of them, but ideally you don't want the hassle. Resistance is always there at a low level, but a real malcontent could "corrupt" the whole plantation and trigger paranoid, though not entirely imaginary, fears of slave insurrections. You can't really breed docility into people (though terror and pain are good ways to extract it) but you can try to avoid the people who look especially unhappy or hostile.

A thing to keep in mind here is that both the sellers and the slaves have some control over how they present to buyers. If a slave has a bad feeling or has heard bad things about a buyer, that slave can present themselves as less healthy, less obedient, etc. There are not things that the enslaver can control absolutely, since slaves aren't bought sight unseen. A few slaves were able to work the system to the point where they appeared unmarketable to the brokers in charge of selling them and were so told to go find buyers for themselves, but that degree of freedom is rare. There's a constant interplay between the buyer, seller, and human merchandise and the enslaved are always trying to get along as best they can in an impossible situation. Having a good eye for slaves, in the antebellum US, is similar to having a good eye for horseflesh: a skill to which any competent man on the make aspires.

The last category isn't as present through the whole antebellum because of legal changes, but is also a factor.

Place of Origin

When slaves were still imported from Africa in large numbers, their imagined origins played a role as well. Slaves from certain parts of Africa were stereotyped as good at particular kinds of labor, more likely to submit, etc. Thus we have instructions from enslavers to their agents to seek this sort of person, but avoid that.

Which isn't to say that they could really tell the difference. Slavers would often have human cargo from diverse African locales, both as a result of multiple stops on the way and the network of slave trading and raiding within Africa. They would market their cargoes with an eye toward profit rather than geographic fidelity and, even if they didn't, the "best" slaves per the then-current received wisdom might have been picked over at previous ports of call. US ports did get some direct imports from Africa, but many slavers would call in the Caribbean first.

After slave imports legally cease (and then really cease about a decade later, bar a few exceptions) there are still occasions where enslavers prefer to avoid slaves that hail from certain locales. The most famous case of this is a general concern about Virginian slaves in the wake of Nat Turner's revolt. Louisiana required registration of all slaves brought into the state from elsewhere in hopes of filtering out Virginians. It's not the quite the same thing as preferring slaves from a different part of Africa, but pretty close. Ultimately the hunger for labor beat out such concerns.

Sources

Many Thousands Gone by Ira Berlin

Soul by Soul by Walter Johnson