My battery capacity is decreasing very fast,... moreover now I can charge with a charger 5.2v and0.5mA (it isn't from nokia) but I can't with a nokia (AC-3X) 5V and 350mA

Months ago I bought a new battery in ebay, but the real capacity was less than half of my original nokia battery instead of the capacity printed in the battery ...

I am afraid to buy a new one in internet because there are cheap and more expensive but how could i be sure the it is a good battery? have you bought any?

And buy a original nokia in phone shop is too expensive...