IMO calling everyone that sells MSFT MSDN keys a scammer is only going to make is easier for scammers to scam because they all have negative trust, so it will be more difficult to differentiate from the ones who are currently acting honestly from the ones who are outright scamming their buyers (eg receiving BTC and in return give nothing, ever). This has resulted in many people scamming while pretending to sell MSDN keys, even after they have negative trust.
This is a good point, perhaps a neutral rating should be put on in place of a negative to inform people that the keys they are selling will likely stop working if the seller is caught. If every user looks the same simply due to what they are selling it would be difficult to point out those doing it legitimately and those not.
Maybe a sticky could be useful in the Digital Goods section warning people about the dangers of buying MSDN keys, rather than tagging all users doing so.
However, this case is valid. Kapitoshka deserves the negative trust, simply due to the fact that he handles bad situations like a child. Anyone doing that shouldn't be trusted with any money until they grow up.