They're fakes on eBay, without doubt.
That's not to say they won't do the job, they just aren't as high quality as the proper ones. There are some cases of fake cards that have been formatted in a way that makes windows think that the capacity is much higher than it actually is, but I think these are rare.
I just bought a 2gig Sony High Speed from eBay here in the UK. The only way you can actually tell it's fake is by the heat seal on the packet. Official Sony ones use staple-shaped heat-seals, whereas the fakes use circular glue dots.
They're probably made in the same factory anyway...