I've bought from Vestiaire a few times, always good service and nice to be sure on the authenticity.
I have also used Authenticate4U from a private sale as I was buying a Chanel bag and wanted to be 100% I wasn't spending so much on a fake, I was pretty confident it was genuine from checking it against a friend's version and I was right. I got it for a steal (in comparison to usual prices for second hand Chanel bags I realise it was still a hell of a lot of money) as it didn't have dustbags cards etc.
I've got a few things off eBay too, including mulberry bags but it takes a lot of perseverance to find the bargains. I'm confident buying from brands I know and can generally spot a fake a mile off. My Chanel was so cheap because people want receipts/authenticity cards/dust bags etc but often the people who have enough high end stuff to sell on for a cheaper than average price don't take very good care of stuff like packaging because they see it all the time as they buy so much stuff, it's people who have had to save and wait for a special high end product save the packaging and treat it like something precious and care about the resale value. Plus having the packaging doesn't guarantee it's a genuine product as if they can fake a bag they can sure as hell fake a card/dust bag/receipt. Anyway off on a tangent, but there are real bargains to be had on eBay and you can always get your money back on PayPal.