I've bought and sold on Ebay, trouble-free purchases over the years including Canon 40D, Sigma 30mmm, Sigma 10-20mm and Sigma 105mm. I've always been careful to apply a bit of common sense in checking out the buyer. At least one lens came from a very low feedback seller (<10) that noone else would bid on, but it just felt right reading the description, checking other items and asking questions.
It is easier being the buyer than the seller, Ebay/Paypal favour the purchaser in any dispute.
The occasions when I have run into problems with online purchases were non-Ebay online shops, where I once didn't do the reputation Googling I should have done. Paypal to the rescue though, and full refunds arranged despite later finding out that the seller in question had a very bad refund reputation. Paying by Paypal and using a credit card to pay Paypal gives multiple layers of buyer protectio - but you should always do the basic checks yourself (Googling the suppliers name and "review" and "problem"/"problems", etc).
Amazon is the gold standard for online purchases, but with care a buyer can get just as good a service buying from Ebay. There are more careless Ebay buyers than there are dodgy Ebay sellers - there has to be, othewise the dodgy sellers couldn't make a living!