Same as aardy above - I’ve built up a number of SSDs all from Amazon UK - mostly Crucial, but some SanDisk - all have been legitimate and worked fine so far (touch wood) - some I’ve had a number of years.
As the other posters have said, I always check it’s being sold by Amazon (or the manufacturer perhaps), rather than some random seller. Amazon is good at taking things back if they turn out to be faulty, or not as advertised.
I will add I’ve had SSDs in both normal “retail” boxes and Amazon’s “Frustration Free Packaging” which is basically a more eco-friendly brown cardboard box usually, rather than a pretty printed box - and they’ve been exactly the same, aside from the box, as the retail packaged product - coming with the SSD, mounting bracket, paper-leaflet, and I’ve not had any issues with any of the SSDs regardless of packaging.
Hope this helps .