No disrespect to Knuckleball, but there is no way I would buy a portable battery from ebay unless I could be very assured that the seller was established and reputable.
The problem is battery begin to lose their maximum ability to hold a charge from the the date they were manufactured.
Their shelf life is give or take 3 years, but will last much longer the faster and more consistently they have full charge cycles.
Go to a reputable physical reseller near you and buy your battery. Plus take your charger with you so that it can be checked I doubt if it is faulty.
PS they are expensive because, what else can you use, it is just like inkjet refills / replacements.