I could be wrong, but I think the dvx has a button to let you turn off the LCD screen so that you can save battery but also access the buttons behind the screen. I haven't used a dvx in about year so i forget exactly how it's done. That could be the problem
is the screen backlight on? or is it just completely black and off? if you could post a picture or a short video clip, I could probably help you out. I'd contact the seller also and ask him about it in the meantime.
I've never shot DVX before but on the HVX there is a black button on the back kinda near the battery that switches it for recording mode to playback mode. Look for it, could very well be your solution.