Need Help with buying a cellphone off eBay?

I'm using Fido at the moment, and the phone im buying is also from Fido, which is a motorola z6w, the 1 im currently using now is a Sony Eric son 300i, can i still use the same SIM card? If it said GSM on the description of the motorola, that means it would work rite? Or does it have to say GSM unlocked, it only says GSM.

Note : If you buy unlocked phone, you can use any GSM sim cards with it . If you buy a carrier locked phone, you have to unlock it by using unlock code. You can get it from any of the online vendors like ProUnlocking.com to unlock it .