SRL Archive - Help

So im getting confused with all this unlocking now. My sister wants an iphone 4S. e have T-moible which is GSM. If she buys a Verizon ipnone 4s with a BAD ESN can She still use it on Tmobile. Can you even use a CDMA iphone ofor Tmobile as they do have a simcard slot. Or can she only get the ATT pnone and unlok it. what about the gevey sim card, will that work?

She cannot use a verizon one with T-Mobile. BTW, never go for bad ESN's.
If you have to get something, get this: http://www.ebay.com/itm/Apple-iPhone-4S-16GB-Black-AT-T-Smartphone-/150942223286?pt=Cell_Phones&hash=item2324db8bb6
Edit: Or this: http://www.ebay.com/itm/Apple-iPhone-4S-16GB-Black-Unlocked-Smartphone-internal-working-screen-/330824016501?pt=Cell_Phones&hash=item4d06a5e275