Unfortunately, it sounds like the pressure sensors in the Wii Balance Board are completely busted. Even if you weren't broke, I don't believe Nintendo fixes Wii Balance Boards.

As for the cause of this, my guess would be Gamestop. People will trade in broken hardware to Gamestop all the time, but Gamestop's policy on that is (paraphrasing): "As long as it works, we'll buy it." So really, the best thing to do would be to just buy a new Wii Balance Board when you have the money, instead of playing russian roulette with Gamestop.

Well technically, Gamestop doesn't sell "new" games. When they get a new shipment of games/hardware, they unwrap the new games and stick them on the shelf. Even though they usually put the hardware back in the box, employees drop and mishandle their games/hardware all the time. So it really wouldn't surprise me that a "new" Wii Balance Board of theirs is broken.

This page (click here) has troubleshooting, for what it's worth. I'd also make sure the floor is level. If all else fails you can call Nintendo at 1-800-255-3700 for more troubleshooting.