I have - like most of the 3rd party seller/auction sites that's very dependent on the seller. I've gotten a couple great pieces but I've also gotten some very obviously synthetic pieces that weren't sold as such. Do your due diligence with questions for the seller prior to purchase since international returns are more or less not an option
As with any large multi seller format, you take your chances, especially on auctions if present. Just like Esty and e bay, but they have good customer protection. It all comes down to the seller.
One more thing that I did not elaborate on with these large multi seller sites of gemstones. They take a big commissions from the seller. Expect price hikes of 15% to 30% more than what you would pay the same seller through PayPal. The seller has to make up for the fees charged to them from sells.