Can I buy an iPhone 4 on Craigslist and use the Walmart straight talk plan?

Right now i have verizon and a stupid flip phone. I want to buy an iPhone 4 on Craigslist for cheap since my 2 year contract isn't up yet. Then I want to get the walmarts 45 $ straight talk plan with the phone. Is that possible. Do I need to buy an unlocked iPhone?

If there are any better deals you know of please feel free to let me know!

This video seems to answer almost all your questions haha.

It is better to buy an unlocked iPhone. But it won't be cheap. If you are buying a locked one you can get it unlocked from here http://www.myhtcunlock.com/unlock-iphone/

Same issue I'm having. Good luck finding a SIM card so you can actually use the service.