Not really, purse.io does not facilitate fraud more than Amazon itself does. Purse.io just connects people who are willing to buy bitcoin using Amazon gift cards for a premium, and those that would like to purchase products on Amazon using bitcoin.
Actually they don't defend this model. Some of their leading heads were hired as consultants by the Venezuelan government and that's about all of the relationship that has existed between them.
There's no objective answer to that, therefore no such "limit" exists. Just pick one that looks reasonable to you, continuously review its validity through the course of your life.
Proof of work burns money and I guess users won't pay for some content's reputation score, there are no incentives to do it other than altruism. POW-based currencies work well because you can exchange the points for stuff or services, not because you can spend them within the system (proof of stack does exactly this indeed and its long-term reliability has been questioned many times).
Perhaps is just me, but I don't think the points in the list of "unsolved capitalism problems" are that hard to solve. Going quickly over them one by one:
Those unable to work are destitute -> People help each other naturally, if a family or a friend has problems you help him/her. There's always something useful anybody can do, it needs not to be a physical task nor require top-tier cognitive skills.
No public goods. No legal system, police, defence, education, or roads. -> "Public" property is just collective private property, we can still have those by associating with peers we choose, also known as voluntarism.
Over-exploitation of shared resources. -> If there are no public goods then there are no shared resources but ignoring that line of reasoning, everything should have a human owner, just to protect it from other humans. Autonomous machines also may own and protect places that are uninteresting for humans.
Cartels and monopolies lead to inefficient resource allocation. -> Cartels and monopolies can't survive consumer association and boycott, if they do, then prices must be fair again.
Information asymmetry leads to inefficient resource allocation, e.g. over-priced goods sold to unsophisticated buyers. -> Can be solved with better, freer education and software (electronic personal assistants).
Significant resources allocated to sales and advertising. -> Augmented reality ad-blockers could probably solve this once and for all.
Ownership of intellectual property restricts use of best-practice processes. -> If there's no global enforcement, then virtually there's no such thing as intellectual property.