In my 6 years on eBay I can honestly say that in general Powersellers give much worse service than 'normal' sellers.
Generally they are too busy and cannot cope with the volumes of stuff they are shifting and simply cannot keep their eye on the ball so to speak, so it all goes to crap.
Approaching 700 items a month is a LOT of selling and would take your life over totally.
I'm a Powerseller and have been for ages but I never display the logo cos it puts people off IMHO. I shift around 150-200 parcels a month AND work full time and I run my ass off keeping it together (but I never let anyone down....ever)
I'm not sympathising with that dude, just offering an opinion.....plus if I saw his feedback I would never buy anything off him in the first place.