If we're attached to the idea that everyone in a society should be able to voice their opinion, then it matters if they feel their voice is their own.
You ask, "if we want people to buy us...what's wrong with that; what does that really mean?" Now working with refugees I can assure you that millions of people are trafficked across the U.S. and the globe, as sex workers, as migrant workers. They are slaves by another name.
While you might be able to afford the idea that some people would want to be considered as simply a commodity to be purchased, no different from a sack of flour, I can assure you millions of people don't enjoy that luxury. Here I will quote the International Rescue Committee (where I am currently doing an internship):
Anywhere from 700,000 to 4 million persons worldwide are trafficked across or within national borders every year. Virtually every country is affected by trafficking, whether capitalized by traffickers as a source, transit or destination location. Generating roughly $7 billion to $10 billion annually, human trafficking is the fastest growing global criminal industry, with high profits, low risks, minimal capital investment, and a "commodity" that can be used over and over again.If we're reduced to "just commodities sold in a market" then we are faced with conditions that would undermine the validity of democratic or republican representation.
And, let's face it, some sort of Libertarian utopia à la Ayn Rand would have to condone this burgeoning commodities market above described, and the existence of this market eats away at a notion of a commonwealth like an acid.
While it's true we might reduce all of life to quantiles of energy exchange, this would ignore and abnegate the other 98% of life that is qualified by how these exchanges occur. To reduce living to a quantifiable exchange is to literally live without meaning.
Why eat anything other than high-fructose corn syrup if the purpose of eating is simply to facilitate the exchange of electrons? By the way, what's been among the most subsidized industries in the U.S. for going into 40 years now? Corn. Guess how the Federal Poverty level is determined - by how much it costs to acquire calories. Guess what the poorest people eat in the U.S. That's right, food slathered in high-fructose corn syrup, because it's cheap.
This is what Marx stated; in Capital, vol. 1:
"As use-values, commodities are, above all, of different qualities, but as exchange-values they are merely different quantities, and consequently do not contain an atom of use-value. If we leave out of consideration the use-value of commodities, they have only one common property left, that of being products of labor."
Why would a capitalist ever want to eradicate protections if it were possible to ensure that some portion of the population was always willing to be sold on the marketplace so as to escape the wretchedness of life unprotected by the commonwealth?