I've owned a few MS refurbs without serious issues. There have been a few occasions, though, where I had to return them for replacements. They once shipped me a refurb SP3 when mine died, that was totally still broken. Needless to say it was another week before they got me a good replacement, but that one was in perfect condition and functioning great (my mother now has it).
As for eBay sellers... honestly, just be careful about who you deal with. There aren't that many who scam these days. Not really. It's gotten quite difficult to pull that off. Counterfeit items are probably a worse issue with some sellers (fake microSD cards, gift cards, etc). The whole "out of stock" BS is something eBay has sought to get rid of for years, but they refuse to go after the really high volume sellers who still do it. Cancelled transactions hurt their standing with eBay (this is automatic, behind the scenes), but it's up to eBay to enforce their own rules (and they're known not to -- serial bid retractors are permitted to retract hundreds of bids for bogus reasons without punishment). That said, I've bought tons of stuff on eBay without incidents like what you describe. And again, eBay's buyer protection makes it relatively easy to get a refund -- really, try it sometime when you're unhappy with an item (don't do it to a decent seller, though, it definitely hurts their standing, but if the seller is a jerk, file a claim). I'm not exactly trying to defend eBay here, but you guys are making it sound like all sellers there are scammers. They aren't. Just read their feedback and don't buy from ones where folks complain about cancellations and similar situations, usually it's habitual with the bad ones. It isn't that hard to steer clear of the bad sellers. Less than 98% positive feedback? Don't buy. There's no reason for that much negative feedback.