Ef-s focal length

Hello all, I am a little confused.

The local seller here said me that when you have a EF-s lens, it can only fit on croped boddys, so the focal length of these lenses is allready converted. So, the 18 to 55 would be not a 29 to 88.

Neverteless when i watch the internet, they still talk about convering these lenses.

So, can you guys help me out, and tell me the real truth about the focal length in EF-s lenses.

Thanks in advance

Telhma