There are a ton of fakes (especially RTT), but yours is authentic. Counterfeits are not allowed on ebay, but unless it is reported (if people know they are looking at a fake) they sometimes slip through. Ebay has really clamped down, and Tiffany has VERO reps that are supposed to look at listings before they appear for everyone else to see, but I don't think they police perfectly. I always report when I see a fake, because I can't stand the thought of someone paying good money for what they think is Tiffany. But you can breath easy, yours is authentic.
Oh I forgot to ask how you can tell the difference between authentic and replica on the tag stuff? That way I can week through when looking for a bracelet.