I have only purchased from BG Stones, several times, and have been very happy. That said, he's located in Thailand and I am in the U.S., and as an individual consumer there are no reliable insured shipping options available to me if I wanted to return anything.
So, if I were contemplating an expensive gemstone purchase, I'd look at each of your 4 vendors and check out their locations, their return policies, whether they provide return shipping labels with their insurance covering return (even if it at your expense), what kind of gemstone lab reports they may provide, etc. Pick the vendor(s) with the policies that best work for you, and then search/find/reach out/wait for what you're looking for.