Some of it depends on when one's ancestors arrived and where they went in the subcontinent. Slave trading to the Cape of Good Hope tended to be the province of either VOC merchants (and many enslaved people were in fact simply exiles from the East) or perhaps some from the WIC or independently. I'm not aware of many, if any, based in Cape Town. The VOC had a prohibition on the enslavement of Khoekhoen and San, which did not prevent their subjugation and use as effectively unfree labor within the Colony, but did prevent a commercial export market from forming of the nature you might be considering.

Internally, it's hard to say. The total number of enslaved people recorded at the time of abolition in 1834, given in James Armstrong and Nigel Worden's still-vital chapter from The Shaping of South African Society 1652-1840 (1989!) notes that from 1652 to 1795 only six ships carrying captives came to the Cape from elsewhere on the continent plus an unclear number of coasting vessels from what today is Maputo, totaling about 1500 enslaved people, while many more came from Madagascar and points further east (112). Many of these captives, especially from other VOC lands held as exiles, were owned by the VOC itself--including many of those forced into sexual slavery. The total number of slaves coming in, registered, was south of 5000 across a century and a half. Again, this does not mean the actual number wasn't higher!

The total number of people manumitted in 1834 under law (leaving aside the practice of apprenticeship--which is still captive labor, but not precisely known) was about 40,000, across 681 owners. (135-136) Most of these were in the Cape division, and most held only 1-5 people. Again, 'soft' captivity or employment of Coloured and other farm labor on exploitative terms (not slavery, but also not free) was far more common.

Only a few Cape residents took part in a commercial trade, and only with the approval of the Company. If your ancestors were among them, the records would be sitting on Roelandstraat in the old colonial prison (now the Western Cape Archives). I can't speak to the demographics there; the smaller scale sale of enslaved people, however, was supposed to be formal but in outlying areas like Graaff-Reinet couldn't be, but I doubt that's what you mean by 'slave traders.' Cape farmers in the Dutch era required subordinate (meaning almost always Khoe, metis, or African enslaved) labor in order to make use of the excessively large farms they held, but not all realized that goal.

In later eras, however, there is a messier wrinkle. The Boer Republics were the result of the Treks, which happened in part because of a belief that the Colony was undoing their way of life by not backing up their expansive claims, undermining slavery and giving rights to free Khoen, and bringing in English-speaking settlers in numbers dangerous to the community (even though they did not blame the settlers for this). The British government at the Cape was doing all of these things, but in going inland, nearly two decades of migration and conflict ensued that ended with the Republics agreeing to abolish slavery (among other conditions) in order to set up their own self-governments in 1852 (Transvaal) and 1854 (Transorangia) with minimal British interference in theory.

This does not mean that slavery actually ended; in fact a significant traffic developed in the far north, in Zoutpansbergdorp (Schoemansdal), aimed outward to Maputo (Delagoa Bay). Jan Boeyens' essay Black Ivory (or his longer Zwart Ivoor) gets into this practice and the complicity of Boers in Schoemansdal, Ohrigstad, and Lydenburg among others in it, as well as that of quasi-prazeros like Albasini (the Portuguese 'chief' of the Shangaans). It was a messy era with mixed African and Boer agency as well as conflict, and if you read Afrikaans, Boeyens' work is very valuable especially his Verhoudinge tussen die Venda en die Blankes, 1864-1869 (Argiefjaarboek vir SA Geskiedenis, 1990). The traffic in this slavery did reach down into the Boer Republics as well, with President MW Pretorius facing a scandal in 1870 over the receipt of two young 'Apprentices' who were clearly part of that traffic. To this day, some black South Africans struggle with lost heritages via ancestors' enslavement as children, but we don't hear much about them.

So basically, the answer is 'maybe.' The chance they were active traders of slaves, as an occupation, at the Cape is very small; in cases from the north, Schoemansdal was never more than about 100 people in size, which is tiny even in comparison to the very small population of Boers north of the Vaal in the 1850s and 1860s. (Venda annoyance at their conduct and claims, together with the refusal of other Boers to come to the aid of a settlement they knew was engaged in the slave trades, led to the abandonment of Schoemansdal in 1867.) The chance that they were somewhere in the slave economy is far, far higher (and the chance that they benefited somehow from the exploitation of such labor is virtually certain, as is true of whites in most of the Anglo world), but that is not your question. Captive labor often existed outside of the specific form of enslaved people, which was no less abject but did escape easy detection. The only way to be sure would be to do the genealogy, and see if your ancestors show up. German surnames are not clear indicators; although a big wave of Germans arrived in the 1840s-1870s, many came earlier, and many were very poor such as the British German Legion which took up smallholdings near the Amathole mountains in the late 1850s as a standing settler militia. There is ultimately not enough to go on here.

I would suggest you might start looking up ancestors in the National Archives's online registers (NAAIRS). Start with KAB or TAB, depending on whether the Cape Colony or the SA Republic, and do some searches, recognizing of course that old Dutch was not exactly standardized. You may want to consider a road trip. Definitely do read Armstrong/Worden (or any of Nigel's books) because they'll give you more data than I could distill here. This has to be a bit quick and dirty (and rambly) because I had about 20 minutes to work with, so I can try to handle follow-ups.

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