There are quite a few. Real Deal Collection is one where you know what you're getting is authentic. HG Bags is another but they don't have a great selection of pre-loved. Fashionphile, The RealReal, Ann's Fabulous Finds, Trendlee...everyone will have their favourites. Beware of Tradesy, they do have a lot of authentic Bals but I've also seen quite a few fakes on that site and they don't pull them if you report them. Some sites will also be more expensive generally than others. Personally I get a lot of mine on eBay but I have them authenticated here first. If you look at the authenticate this thread on tPF you'll see a list of photos that would be needed to have a Bal authenticated. When in doubt, authenticate here before you purchase, hope this helps! (PS, take some time to look through the reference sub forum, there's a ton of good info there!).