Simply put, “It's a voluntary transaction” can never be a sufficient argument for why something ought to be legal.
Eg. I'd rather be a slave than starving on the road, I'd rather be dead than in some particular irreversible painful medical condition.
The only actions which are ethically wrong in my book are hurting someone else or damaging / stealing their property. Incidentally the government breaks these rules legally everyday (eg. taxation, wars, spying on people).
And you would throw it all away because of some personal libertarian principle? This isn’t only about what you would like to be able to do in any given moment, it’s what this ability does to society as a whole. Your ability to someday be able to sell yourself into slavery has some value (for the principle of the thing, if nothing else), but it is certainly not worth the above-mentioned effects on society.