If the device you're looking to purchase is described as having been "flashed," "modded" or "running a non-stock ROM," don't purchase it. We may not be able to get the phone activated at all.
The difficulty with devices like this is that you'll probably be able to add them to your account (assuming the ESN is good), but you may not be able to get the phone activated. With a bad ESN, you at least know the phone isn't going to work for you right away. With a flashed or modded device, you may end up with a device you can't use that you also can't get a refund on because your return window has closed while the device was going through the BYOMD™ process.
There are probably some lucky folks who have managed to get a flashed or modded device working with Madstar Mobile, but we can't guarantee you'll be so lucky. It's probably best not to buy a flashed or modded phone.