Interesting findings today.....
Got home and both packages had arrived. My Sennheiser SX300's from eBay (£7) and my Sennheiser SX300's from Amazon (£14).
First off, different packaging, but both feel and look like the quality you would expect. Aesthetically, they are also identical, at least to the naked eye.
Sound wise, I tried the eBay ones first, and the sound is bl00dy good! Until it is turned up a bit and then the treble starts to get a bit harsh.
Then I tried the Amazon ones. And what a difference! The sound is so much more balanced, less bassy but so much more controlled.
I have no idea if either are 'fake'!
In summary, for the money, in isolation I'd be well happy with the eBay ones, if maybe a tad bass heavy for some lyrical tracks. So for £7, I can reccomend them!
But if either were fake it is likely to be the eBay ones. And for £14 off Amazon for these little things, I am massively impressed.
Just got 4 pairs of headphones now!!