The eBay ratings - despite being numerically the most egregious - are probably the most helpful. 99.6% ostensibly says that there's a 0.4% chance of you having a bad experience. Star ratings don't really convey any meaning.
I'd definitely adjust my baseline "acceptable percentage" based on what I'm buying. If I'm getting a pack of USB cables for $0.05 each, I'm not going to be too fussy about the rating. I'll take a 2% chance per cable of them not working, that's what I'm paying for. If I'm buying a new GPU, I'm going to be a lot pickier about it.
I'd definitely adjust my baseline "acceptable percentage" based on what I'm buying. If I'm getting a pack of USB cables for $0.05 each, I'm not going to be too fussy about the rating. I'll take a 2% chance per cable of them not working, that's what I'm paying for. If I'm buying a new GPU, I'm going to be a lot pickier about it.