Are old unused macbook batteries ok? I'm considering buying a Geniuine Apple Macbook Pro (2007) battery on ebay.  This is NOT an OEM knockoff, and the battery is still sealed in original plastic, but I have no idea when the battery was manufactured.

If the battery was made, say 5 years ago, is it likely to perform almost as well as one that was recently manufacurerd?
The short answer would be no. A 5 y.o. battery won't last as long as a new(er) one. However, depending on if the original owner followed Apple's care instructions, and it doesn't have a high cycle count, it may not be much lower.

A lot of it depends on the care the battery was given. If the battery has just been sitting in a box this whole time, it's performance may be lower. Batteries need their "juices" (electrons) flowing in order for them to stay healthy. 

On the other hand you have cycle count. A battery only can handle so many cycles before it wears out. Simply put, the higher the cycle count, the lower the performance.

Apple recommends storing a battery at 50% for extended periods of time. If it's fully charged or discharged when stored, this will again affect battery life.

Here is a comprehensive guide to Apple notebook batteries.

Of course, all this doesn't count any engineering changes that may have been made for the better in newer batteries.