Can I buy an iphone from Craigslist eBay etc and then set up a plan with virgin Mobile on it?

The phones on their website are almost 500 dollars each. I can buy one on another site for less than 200. It's much cheaper. Do I have to buy the phone from virgin mobile to use their services?

You need a Virgin Mobile phone to use their mobile network, you can look on craigslist for a VM phone, but that is still very risky. Many phones sold on CL are stolen and are blacklisted (blocked) .

Look on Amazon or eBay, but there are no really cheap iPhones due to the high demand from teens.

You can buy the phone from outside your carrier and have it activated on your number. Make sure the phone doesn't have a bad ESN before you make purchase on eBay or Craigslist. If you want to buy any iPhone without contract safely and at reasonable price, mail me.