A couple of things:

-Ebay is full of batteries (and other goods) which look genuine but are actually counterfeits, so don't take sellers at their word just because they claim it's a real Nokia battery.

-Unofficial batteries may well work fine but they tend to have a much higher risk of short circuiting and blowing up, especially the cheaper models.

IMHO batteries are one of those things where it probably is worth going for official models from reputable retailers.

(Memory cards on the other hand are totally different, never buy Nokia-branded memory cards because they're stupidly overpriced.)