A fence is providing a few semiseparate services:
1. They will buy stolen goods. You get a lower price, but you don't have to face nasty questions like "how did you get this?"
2. They may assume the risk of policing. If you stole something surprisingly important and it gets thoroughly investigated, the fence goes to jail, and you don't.
3. They locate people who want to buy stolen goods. It's easy to conceive of this as a service they provide for their own benefit, after buying your stuff (#1), but in reality you can't really do either without the other one.
Amazon commingling inventory lets it provide service #2. The police can establish that a stolen object was delivered to an Amazon customer, but if it came from a commingled pool, they can't establish who sold it.
But Amazon's whole concept is to provide service(s) 1 and 3. This would make it fairly strongly parallel to a fence who, when the police show up, lets them know where to find you. But even then Amazon is still doing the largest part of fencing; usually the police don't show up. The hard part is locating the customers, and that's what Amazon does.
At least it's Amazon losing money then rather than Amazon benefiting from crime against competitors.