It is legal in the US. In fact you can potentially make a decent bit of money if you're paid $70 per donation and can donate 100 times per year [0]. Don't know if it would be worth the time to travel to the centre and wait in line, for many HN users.
You could sell your plasma twice a week in the town I went to college. We also found that it took significantly less alcohol to get drunk if you had nearly a liter less plasma running through your veins, so it would also save us money in the evening!
My understanding, at least where I live, Florida, is that plasma can be sold, but whole blood must be donated. I still get a $25-$35 gift card every time I donate a pint of whole blood (as close as possible to every 8 weeks).
[0] https://www.goodrx.com/health-topic/finance/how-much-donatin...