By studying the "slave schedules" of the 1850 and 1860 US census and period documents of individual slave owners, it is possible to gather actual data instead of speculating. It is possible to compare the number of enslaved adults and children enumerated on one census, and the next. Some slave owners kept other records including names, marriages, family groups, and housing assignments. All of these records combine to offer a better understanding of what occurred and how. For example, on one relatively large plantation (est 1845 ish) in North AL:Plantation management could follow a similar course, when demanding a high labour burden (eg. to work more land than usual per capita) from their enslaved workers and rigorously "sell them off" as soon individuals showed signs of strain or were no longer able to deliver the demanded performance. With slave prices high this policy could maybe also be profitable - even though it had terrific repercussions on people and especially their families.
The 1850 US census "Slave Schedule" shows 18 children and 23 adults
Age 12 and under = 13 (8 female 5 male)
Age 13-17 = 5 (3 female 2 male)
Age 18-35 = 19 (10 female 9 male)
Age >35 = 4 (3 female and 1 male age 75)
1850 TOTAL of 41 enslaved people
The 1860 US census "Slave schedule" for the same plantation shows 37 children and 33 adults
Age 12 and under = 27 (15 female 12 male; of these, 25 were born after the 1850 census)
Age 13-17 = 10 (4 female 6 male)
Age 18-35 = 28 (20 female 8 male)
Age >35 = 5 (2 female 3 male)
1860 TOTAL of 70 enslaved people
It's important to your point about "early slavery" to note that the initial, and some subsequent, enslaved individuals were obtained by this planter through inheritance. The number of enslaved people increased, over time, by nature. I've compared the ages of people from one census to another and there is no evidence that 'sell off' was part of the management of this particular plantation. After studying extensive records - from inheritance to the end of slavery - there is no evidence that any enslaved person was ever sold from this plantation.
It's also worth noting that only one enslaved person was ever 'purchased' by this plantation owner - the increase in number of enslaved was by nature, and by additional inheritance. The single 'purchase' was one enslaved man who was inherited by the brother of this planter, used as collateral by the brother, and purchased back by this planter. This was done to prevent the enslaved man from being auctioned to satisfy the brother's debt. The records are extensive and extremely detailed. If sales or other purchases had occurred, there would definitely be records.