My question is broader than the one in the title and It's a little clunkily worded in the first place, obviously slavery is incorrect no matter who is enslaved.

An example of what I am asking:

A "free" person goes to a slave trader looking to buy a slave, they're not massively wealthy, just need some extra labour or something along those lines. What's to stop the slaver simply taking their money and gaining an extra slave to sell? would a slave crying out that they should be free be listened to by passersby or just ignored?

Broadly what I'm wondering is how did societies that engaged in slavery guarantee (or give the impression) that slavery was not something that could befall the free people of that society? (since I assume that if the threat of becoming a slave was ever present to all of the populace then the whole system would quickly become unpopular)