Deuteronomy 28:68 states “ye shall sell yourselves unto your enemies for bondmen and for bondwomen, and no man shall buy you.” Oh dear, even at a price of zero, supply would exceed demand. (Josephus noted that there were so many slaves on the market when the Romans destroyed Jerusalem in 70 C.E. that many couldn’t be sold even at fire-sale prices.)
Why not buy a slave at no cost? The answer, presumably, is that potential buyers already owned so many low-priced slaves that they believed that another slave’s marginal product would fall short of his or her upkeep. The variable cost of maintaining the slave must have exceeded his/her output. Is there a contemporary analogy to teaching assistants?