That's possibly another reason why some of those batteries are so cheap: Rebranders buying two to three year old surplus stocks from those Chinese manufacturers and reselling them.

Not enough people know that LiIon batteries (and their polymer cousins) deteriorate by simply getting older. That's also the reason why it's not very economical to buy a backup battery "in advance": It'll just die slowly in the drawer.

If you really need to store them, charge them to 40-60% and keep them in the fridge.

Even then the rot will set in... albeit a bit slower.

I just don't buy extra LiIon batteries anymore; I bought one for my P910 almost two years ago and now it's almost as bad as my original primary battery, and hardly ever used (yes, it was a knockoff, so my loss is about USD 5.00).

I go for the external emergency battery chargers. Much less headaches: Cheap replacement rechargeables and mine takes the ubiquitous AA cells. With 4 2500 mAh NiMh cells, it triples the battery life of my phone and almost doubles that of my N800.