I've only used it a few times and am very wary and cautious when looking to buy.
I generally trust the shops that have hundreds or thousands of good ratings but for individuals, I try to steer clear unless there are information on the ad about the person, in which case I try to check that person out. If everything seems legit, then I would bid.
I havn't sold anything off ebay however. I always try gumtree.com.au or here first. And so far I've been successful so no need to use ebay.