Think it is easy to identify the exceptions - but I don't think a system should be premised on the exceptions.

In anycase IMO most decent sellers don't have dozens and dozens of disputes opened against them, they put in place plans to manage the risks - such as writing clear and accurate descriptions, using photos of actual item, packaging well, prompt post and sending via post method appropriate to value etc etc  

How the competition stacks up, within those areas where there is a higher risk of disputes, would serve as a benchmark for a discerning buyers comparison in any case, as it does now with stars and feedback.