While there is surely more to it, this kind of scenario should have been predicted before Internet companies got big. You see, the company can lose real money if there actually is a legal issue with an account holder and they don't act; they can be implicated in crime and be fined and have to spend money on attorneys to sort it out. However, it costs the company absolutely nothing to find, using automation, all complaints against any account holder valid and instaban them. It's cold, hard business. Everyone accused is punished without any resources spent on investigation to discover the truth. The truth here doesn't matter to the company. People don't matter to the company. Only money matters.

So we should start accusing all the biggest sellers until somebody does something more useful?

No. Business should be regulated to protect the consumer and remove this incentive for businesses to unilaterally screw the consumer.

The current situation is screwing a small number of unlucky consumers, but providing a net benefit for the overall aggregate of consumers, due to lower prices and fees.

What your proposing is to screw consumers overall in order to provide fairer treatment for the unlucky few due to ebay having to charge much higher fees in order to cover the cost of fraudsters.

Since buying and selling goods and services online is not a constitutional right, or any right anywhere as far as I know, a proposal to force such a change does not seem like it will pass muster in a serious court of law.

I'm not a lawyer but even to my untrained eye you will need far better arguments to get any traction.