Wage labour is selling your body for a time. Why not sell your body by part?

The time is gone anyway, and barring industrial accident you still have your entire body to sell again afterwards. That is not true of kidney donation — you can't donate a second time¹ and if your remaining one fails you are up shit creak because if you were in the position of being willing to sell one back then you are unlikely to be in the position to buy one now.

Donation of blood and other replenishables is a bit different of course, but allowing open sale opens some bad avenues in terms of people effectively being forced to (which could carry risks they wouldn't normally want to take).

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[1] Well, not practically…

> The time is gone anyway, and barring industrial accident you still have your entire body to sell again afterwards.

Your two statements contradict each other.

In any case, we only have a finite number of days allotted to us on earth.

> [...] you can't donate a second time [...]

You can't repeat your 30s either.