Merely possessing stolen goods is a crime where I am from and is more than likely the same where you are.

Nope. Possession of stolen property is not a strict liability offense under US criminal law. Prosecution requires mens rea.

This depends - if a person goes out of their way not to find out whether the goods are legitimate, it's equally bad.

Either way, if you possess stolen goods, they will usually be taken from you and returned to their owner. If that happened to a lot of Amazon customers, the refund volume would encourage Amazon to crack down on their supply chain.

And many people at Amazon surely do know that they sell stolen goods but decide to do nothing about it

And many people who buy from Amazon know Amazon must happen to sell some goods that are stolen, yet they still shop there. If everyone had to cease buying/selling because they know their counterparty must have some stolen goods then trade would literally ground to a halt.

I don't know if it's a crime if it's just "somewhere we must be buying stolen goods, but we don't know where." I'm not a lawyer but I think knowing that the specific item you bought/sold was stolen would look differently in court than just having some vague knowledge there must be stolen stuff somewhere in the supply chain.