"Forced to sell himself"

Like cavemen had to "sell themselves" to hunt, taking the risk to their lives to not starve.

Like agrarian societies has to "sell themselves" spending their time and effort to manage crops.

Like business owners have to "sell themselves" taking on the capital risk of failure, with no labor laws to tell them what a reasonable amount of time and effort is.

Engels "discovered" that nothing gets done if people don't work.