What Closed_doors said about eBay feedback is pretty much "spot on" -- however....
If a seller has tens of thousands of feedback because they sell a LOT of stuff -- and you'll probably seldom see this if they're mostly selling kinky or BDSM items -- they'll probably also have a large amount of negative feedback as well!
Why? Because a lot of people are idiots, and on the internet where people are anonymous, people often ACT like idiots -- and you can see this same "negative review" nonsense all over amazon.com as well.
If you actually READ the negative comments, you'll often see that they're from people who really don't understand how the feedback system is SUPPOSED to work! I've seen negative feedback for reasons such as "husband didn't like gift," "couldn't figure out how to set up the [electronic device]," "didn't like the color" and so on. (Those are all valid reasons to RETURN the item for a refund, but they're NOT reasons to blame the seller.)
Now, if that last one had been "ordered blue widget, seller sent me a red one" instead, that MIGHT be a valid reason to provide negative feedback, unless the description said "assorted colors, no choice" meaning that the seller just picks a random colored item and puts it in the box to be mailed. "Widget appeared to be a darker blue in picture" might be a reason to return it, but again, not really a reason to give the seller negative feedback.
One of my personal favorites when a seller sells a thousand items a month is "post office lost item, seller sent replacement." It doesn't happen often, but when it does, they should blame the post office and thank the seller instead!
tl;dr: Don't be immediately scared off if a seller does have some negative feedback -- dig deeper and see if it's LEGITIMATE bad feedback or not.