Parshat Ki Savo
In 28:68 the Torah is nearing the end of the "tochecha" section. This is a big warnign containing a long list of horrible calamities that will befall the nation if they stray from the path of Torah observance and dedication to Hashem. The pasuk says, "And you will be returned to Egypt in boats in the way I had said you would never see again and there you will be sold as slaves and maidservants, yet there will be no buyers."
What is the great calamity? After reading about horrific tragedies waiting to befall us such as going crazy, eating your own children, destroyed by the enemy, crops not growing, among many others. After all that, not being bought as a slave does not seem like such a horrible punishment! So what is the big deal that this is the final threat in the warning?
I would like to suggest that this really is a real threat. It is a threat to a persons psyche. everybody wants and needs to know that his life and being has some sort of value. If I cannot even be sold as a slave that means I am pretty much worthless. I am a good for nothing.
By warning us that nobody will even by you as a slave, Moshe is warning the nation that they will be made to feel worthless and good for nothing. That, as we all know, can lead to depression and a horrible state of mind.
That is the threat of the passuk and that is how it fits into the context of the tochecha.