But it has been a couple of years in storage and wasn't tested.
That likely killed off whatever life remained in either battery. My 2003 PowerBook G4 needs either a working main battery or a working PRAM battery to boot. It will not boot if both are dead. My PRAM battery is dead but, fortunately, I was able to get a new main battery from Apple not long before they discontinued those batteries a couple of yeas ago.
Sometimes you can pump just enough life into the PRAM battery to get a boot by removing the main battery, turning the computer off, and leaving it attached to wall power for 24-48 hours. A long shot but it has worked for me in the past.
The trick K Shaffer mentions about disconnecting the battery from the logic board only applies to certain models of the powerBook G3, whose PRAM battery conveniently has a plug that attaches to the logic board directly under an easily lifted keyboard. Most G4s have the PRAM battery integral with and soldered to the right-side USB board--hard to access and even harder to disconnect.
Were I in your shoes, I would first consider contacting the seller, especially if the computer was advertised as "working." As it sounds like it was in a corporate environment, maybe they have others that actual work. Otherwise ask for a refund.
If you cannot do the above, then OWC, whom K Shaffer has already recommended, is you best resource at this point for a new main battery:
http://eshop.macsales.com/item/NewerTech/BAP17AL62RS/
The PRAM battery he links for the 17-inch 1.67ghz PBG4 is the only one they have. I have searched since getting my old PowerBook in 2008 and cannot find new replacement PRAM batteries for any pre-1.67Ghz Powerbooks. The links to batteries for sale on the iFixit site redirect to eBay, where the batteries are described as "used." Used batteries sound about as appealing as used chewing gum to me. However, the battery is supposed to be rechargeable to some extent so, as the prices are now a fraction of what they were three years ago, you may wish to take a chance on one if you are up to the non-trivial task of accessing and replacing the right-hand USB board.
AJ