I once needed a NanoVNA. Bought the cheapest one available on Amazon with next-day delivery. Works well, and I have updated it several times with the "official" firmwares.
They are all probably built on the same (Open Source and freely available) schematics and board designs, so I don't really see any reason to worry. Sellers on Amazon would have too much of a reputation to lose selling non-working items. Plus, here in Europe, Amazon allows free returns during at least 2 weeks, normally longer. So, if I get something that doesn't work, in the next days it's backk to Amazon for a full refund.
