with regard to ebay, you can put certain things in your preferences to get rid of a fair few. e.g. if they have a history of none bidding.. you need a pc to access your preference though, can't do it on a phone.
I have plenty of tyre kickers, prank bidders etc.. just also look up "block bidder list" and you can add em if they don't pay up.
I'd say about 75-80% of folk are fine.. just the scammers, scalpers, mis-describers and postage gougers that trash it.
I just took legal action against someone last year, he never bought off me.. and thought it "fun" to post BS about me on social media, in front of potential customers... lets just say with my costs etc. and his own , it's been a rather expensive end to his 2016.