It doesn't follow that when two individuals are incentivized to transact with one another, that that transaction has a net positive impact on wider society; there may be negative externalities. Additionally, people are not always completely rational and perfectly informed agents, which means that not all voluntary transactions do render positive value to them. Selling your arm is vanishingly unlikely to be an efficient use of resources, particularly as arms are not fungible (you cannot change your mind later and reacquire an arm). An arm is just much more valuable when attached to a person, and if the economic incentives point the other way, we should probably look for how such a perverse situation came about, rather than fire up our circular saws.

Yeah, this is the argument for bitcoin, that if the transaction occurs then it must make sense for society.