I emailed that seller to see whether his battery would fit a sprint diamond. His English is not so great, but I think he means to say that the one you bid on will not fit the DIAM500, although he does have one that will fit and it comes with a maroon cover. His message is cut & paste below for you to see:
Pete
Dear Valuable Customer, Thank you for your inquiries! Not,this battery will fit, but,cover not fitting. We have fit to Sprint HTC Diamond (DIAM500) . cover color is Maroon. Also Capacity is 2400mAh HIGH CAPACITY battery. Total is USD$17.5 (INCLUDE SHIPPING).
If you like,please send to my paypal A/C, paypal A/C is: payment.battery@gmail.com"