HTC Diamond phones experience a common problem known as the “White Screen of Death” which is a critical error causing the phone to become unusable. This is similar to the “Blue Screen of Death” or “BSOD” that Windows users typically encounter during a critical system failure or error.

After doing a little research this is the only conclusion I could come to regarding your question. The person was asking you to make sure and buy them an operating phone, because if you buy it online someone may try to scam you with a broken “White Screen” error phone.

More info on the white screen of death error can be found at the HTC wiki forums here:
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Hope that helps.

There’s probably a difference between the normal White and the Ice White model.