Except what is being “bought or sold” is not anything material. Buy and sell is a synchronous event. I am buying patience with my impatience. I am selling my unkindness in exchange for kindness. Consider the idea that when we are born in the natural we already know evil, we are masters before age one never even having been taught. So all those things we know, such as hate, greed, unkindness, impatience, conflict, etc, we sell all these and buy love, generosity, kindness, patience, peace, etc with the proceeds. We do this until we sell all that we have. The Lord buys all of it from us, he pays the price for our sin. And when we have sold all we have, we have also bought everything in place of it. Therefore, we no longer have the mark and can no longer buy or sell.