There are good deals on eBay for coin type batteries. The same battery you would buy at a local store for 4$ will usually cost you about one fifth of that price. This is a good deal, but the big question is: are you getting a quality product ? is it old batteries that are expired ? How to determine when it was manufactured ?
On the Maxell Brand batteries, there is a date code printed on each battery indicating when the battery was manufactured.
This code is two digits: The first number indicates the year and the second number indicates the month.
For example, "80" means that the battery was manufactured in January 2008. This example is tricky because "80" can be read as "08" is the Battery is upside down.
The battery should be rotated so that the date code is at the bottom of the battery, and it should be read that way.
This type of battery is usually good for 5 years.