It is controversial to put into system letting people sell their bodies to make ends meet.
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…
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.