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The problem with purchasing from E-Bay is you never really know what you are going to get. It could be legitimate, or you could be scammed.

The reviews are positive, and the vendor has a good reputation, so that should provide some peace of mind. (there are also some negative reviews in the last 6 months...)

Personally, I purchase my parts from reputable vendors, like OWC at macsales.com because I a very low tolerance for risk. I'll be curious as to what you decide and how it works out for you.
 
Frankly, I'd suggest you just leave it alone.

If you need "more storage", buy an EXTERNAL USB3 drive (it could even be an SSD like the Samsung t5).
Then, do some "cleaning" on the internal drive, moving the "less frequently accessed" stuff to the external drive.
 
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