Afsalva beat me to it... you just have to be running an IOS version below 5.1 (so if you've got 5.0 or 5.0.1 you can gevey unlock it to run on GSM networks.)
The reason you can do this is the iPhone 4S cdma version has both CDMA and GSM basebands in it (making it a world phone.) With verizon or sprint you can unlock the phone to use GSM internationally, but it will not work with AT&T/T-Mobile. Get the Gevey sim and you're able to run it on any GSM network you want.
I buy broken/used iPhones (4 & 4S) on any carrier! PM me info about your phone and your asking price and I'll get back to you!