You don't have to intend to commit a crime to have mens rea. You merely have to intentionally do all the steps involved in the crime. Ignorance of the law is not an excuse, and neither is intentional ignorance of whether you are breaking it or not.
Pawn shops got the receiving end of this deal a while ago. Now it's Amazon's turn.
Intent does matter, however - plausible deniability would not be a thing otherwise.
Only in cases of strict liability are intentions not usually considered, which mostly include minor offenses (infractions) and a few major ones like statutory rape. Possession of items that are illegal to possess in any situation is another case where strict liability applies, but possession of legal goods that “a reasonable person” would not be aware was stolen does not fall into this case. Amazon sellers sign a contract saying the goods they offer for sell are their legal possessions to sell, giving Amazon plausible deniability as if anyone breaks the law, as the seller would then also be breaking their rules as well.
I think we need to ask ourselves the harder questions of why people are stealing and figure out how to address that issue. Just like how we here tend to condemn technological solutions for social problems, tons of legal regulations can be fairly criticized for a being a legal solution to a social problem.
I'm not sure why the fundamental problem here is that people are stealing, rather than that a known billionaire is willing to operate a business that is known to be an attractive way to profit from theft. You almost certainly can't eliminate theft without predictable laws; no one ever has, and to the extent that it's been reduced, the existence of a reliable legal process has been part of it.
And Amazon can't exactly plead "plausible deniability" in an absolute sense here, because they ought to be aware that they're acting as a fence, and they ought to be aware that their processes are insufficient for reducing the degree of theft that's going on. If they've deliberately chosen to be ignorant of those facts, it would actually raise their level of culpability.
Finally, your analogy of legal solutions to technical solutions fails. In a democracy, legal solutions exist to meet the burden of social expectations. They do not stand outside of society. Unlike democratic laws, technical solutions are usually imposed on a community by people with disproportionate power - laws are created on a one person - one vote basis, but technical solutions are usually created on a one dollar - one vote basis. Therefore, technical solutions are usually not responsive to the needs of a society, and are far more likely to represent someone's One Fantastic Idea to Solve All Problems, rather than a compromise of interests resulting in social norms that can be imposed on defectors.
This is exactly an occasion when existing laws should be used, rather than plea for the devaluation of trust and law by ignoring laws simply because they might be applied against a powerful person.
