Any deal involving something that one party doesn't know or understand is probably unethical. It's also questionable whether the aboriginal people understood the implications of trading their land.
I was hinting at crypto-currency mining which is an activity that would be meaningless if it did not produce results that are rare enough to suffice as store of value. I updated the earlier comment to make that clear.
Here's what you're missing:
What is rare about the results produced in mining is something that we humans would have control over, not the aliens.
Aliens showing up with literal alien technology is one of the few things that might get a cryptocurrency to update their PoW someway or another.
If an alien appeared before me and offered me some crazy never-before-seen four dimensional shapeshifting object in exchange for a bag of coffee, I'd accept it. Was I taken advantage of? I'd be the envy of my friends, for one.
Clarifying edit: I believe the Manhattan example to be unethical, but probably not the alien example. The reason I think so is because I highly doubt (given colonial history in North America) that the trade terms were particularly clear or even that the decision was theirs to make.
Rambling a bit, but here's another edit. The Manhattan trade sounds particularly fishy to me. Imagine that the alien was offering you a shiny 4D object in exchange _for your house_. That would be so ridiculously unwise that I can't really accept that the residents of Manhattan island thought it a good deal.
On the other hand the trinket might really be more valuable than your house as exposure to some technology that doesn't exist in the economy where you live. Or, trading coffee was equivalent to smuggling silkworms out of China. Or, you sold them the Brooklyn Bridge rather than coffee or your house.
The people who sold Manhattan may not have understood the selling of territory, or been good representatives of all of Manhattan's residents, but I don't think beads are entirely the problem.
Napoleon sold inhabited territory he didn't control, and had no intention of wearing all of the gold. He was going to trade it away for useful goods and services necessary to take Britain by force.
What were the plans for the beads?
Let's say you have no prospect of sourcing the same beads (or therefore no need for a deep water port either) yet there is an entire continent west of you that has been depopulated by disease, and the remaining population has even less access to beads.
My point is that vanity and status confer real value on beads and gold, because it makes them tradable goods. Likewise legalistic notions of owning land are different than de facto control. I think the discomfort comes from the dynamics of a deal made when one party has more power.
This is a rather popular myth, but most academics reject the virgin soil thesis as some combination of incomplete and simplistic. Europeans were intimately involved with precipitous population declines where they occurred in North America.