How to transfer sim cards?

I was going to buy an unlocked phone on ebay and switch the sim cards out. I would still be on the same network as the unlocked phone. I wanted to know if this is possible and if i can to do it myself or if i need to take it somewhere. i also wanted to know how to tell if my exsisting phone has a sim card or not in the first place.

How to transfer sim cards?
to find your sim.. take out the battery. I know that Verizon and Sprint don't have sim cards.. if you have an unlocked phone, you can use it with any phone provider just about. Just take out the sim an dput it in the new fone. You can check if your provider is compatible with your phone on ebay.. its somewhere on the page
Reply:If you do have one, it is very easy to do. It is usually located under the battery. Remove the battery, and then you will see the card. It looks like a tiny credit card. It should just slide right out. Remove it, and put it in your new phone. You just have to make sure you have one, and the new phone you're buying is compatible.
Reply:open the back of the phone and then take the battery then the sim card should be a little card that is put in underneath a piece of plastic then take it out and do the same to the other phone and switch their spots