The police won't care and Gumtree, being what it is, probably won't either. But since it's high spec you should get at least £200 back on eBay. Just learn your lesson from this and remember to check these things very carefully in the future. If you need to, meet with the seller in a phone shop so you can compare what he's selling with the display phones to make sure it's legit.
To be fair, the heart rate sensor doesn't work on the actual GS5 either
Good idea about meeting in a phone shop.
If the guy refuses then you know something's wrong.
Great way to avoid scammers.