Slavery is treated as a normal aspect of life for those, like Crusoe, who ventured out into the wide seas. It is a hazard anyone aboard a ship faces, no matter what his race or nationality. The young Crusoe himself, who did not heed his father's warning that this would happen, is soon on board a ship captured by the Moors. He is enslaved. He writes about it both as not as bad as he had feared, yet still terrible enough to overwhelm him:
the usage I had there was not so dreadful as at first I apprehended ... I ... was kept by the captain of the rover as his proper prize, and made his slave, being young and nimble, and fit for his business. At this surprising change of my circumstances, from a merchant to a miserable slave, I was perfectly overwhelmed
However, ever resourceful, Crusoe does manage to escape and reunite with western Europeans. When his fortunes change, he takes no lesson from his distress as a slave but simply takes advantage of the resources provided by his new plantation in Brazil to buy a slave and a European indentured servant to supplement the one provided by his captain. Interestingly, he does not distinguish between the slave and the indentured servants: all are "bought."
for the first thing I did, I bought me a negro slave, and an European servant also—I mean another besides that which the captain brought me from Lisbon.
Nevertheless, the plight of the white Europeans "bought" by Crusoe is far less dire than that of the Negro, as Europeans would serve him only for a limited time.
When he discovers that cannibal "savages" are rowing to his island. Crusoe dreams of enslaving some of them:
Besides, I fancied myself able to manage one, nay, two or three savages, if I had them, so as to make them entirely slaves to me, to do whatever I should direct them, and to prevent their being able at any time to do me any hurt.
That doesn't work, but when he saves Friday, the young "savage" feels a great indebtedness to him and functions as his slave. Friday's plight, like that of the European indentured servants, is helped by racial politics. Crusoe softens to him because he looks "European" in many ways:
he had all the Sweetness and Softness of an European in his Countenance too, especially when he smil'd. His Hair was long and black, not curl'd like Wool; his Forehaed very high, and large, and a great Vivacity and sparkling Sharpness in his Eyes. The Coulour of his Skin was not quite black, but very tawny; and yet not of an ugly yellow nauseous tawny, as the Brasilians, and Virginias, and other Natives of America are; but of bright kind of a dun olive Colour, that had in it something very agreeable; tho' not very easy to describe
Thematically, in the broadest sense, being master or slave is matter of luck. Sometimes you are on the bottom of the wheel of fortune and must adjust, as Crusoe did, to slavery, but sometimes you are on top and can enslave others. This is depicted as the byproduct of an amoral universe in which might makes right, and how much power you have amassed at a given moment determines whether you live in ease or toil. However, complicating this is the fact that looking more "savage" implicitly made the slavery situation worse for some and looking (or being) "European" made it better for others.