@letscleanupmycupboardswrote:
So I'm also guessing smaller volume sellers, for whom one breach or negative would mean dropping below minimum, will be at risk of buyers using this as a bargaining tool. So it's another thing in addition to worrying about returns or disputes for 6 months after a sale and already huge fees. Good way to drive sellers to perform or just to give up selling. Nice work eBay.
Fortunately, eBay do have a small measure in place to prevent low volume or casual sellers from being impacted - negatives don't actually affect a seller's standing at all, and the things that do have to come from at least 2 different buyers (4 if we're talking canceled transactions for being out of stock - the thresholds are lower for Top Rated status, but losing that and going to 'above standard' - the default - won't result in the additional fee).
The reason a transaction is being canceled is selected by the seller, so regardless of what causes it, it's up to them what they choose and whether or not they get a defect out of it, and the only other thing that results in a defect that can make a seller fall below standard is having a 'case closed without seller resolution' defect (which is where a buyer opens a request, and then escalates it to eBay to make a decision and it's found in favour of the buyer - the seller gets 5 days to resolve the case before the buyer can escalate it, for whatever reason they want to, and there is only one way to 100% guarantee that the buyer can not escalate a case).
In other words, eBay are encouraging sellers even more not to fight them, in a "pay now, or pay twice later" way. 
