Can iPhone without a sim card be flashed?

I just bought an iPhone from my friend and it's with at&t and I was wondering that if I take the sim card out and go to a place that flashes phones to Cricket would they be able to do it. Also the phone is still turned on with his at&t account (his text messages are still coming through and it can dial out) so will that be an roadblock in getting it flashed.

Another question I have is that if I can't get it flashed, can I go and sell it to a place that buys iPhones like Game Stop or one of those mini stores that be in the mall. Will they tell me that since it's turned on, they can't buy it or since it's not mine, I can't sell it.

Yeah sure