You conduct a battery test by loading up a 50+ hour playlist of music onto it. Plug in a standard set of headphones (the factory Apple earbuds will work just fine), set the volume to 50%, start the playlist (it has to be a playlist), and walk away. Don't touch the iPod until it completely turns off. Plug the iPod back into your computer, look at the last played song in the playlist, and tally up the playtime (you can do this in iTunes rather easily by highlighting all of the songs that were played and it will display a total playtime at the bottom, I'm sure foobar2000 has something similar).
The iPod classic's battery is rated for 36 hours of audio playback. However, those battery estimates from Apple are on the conservative side. You should be getting about 40 hours of total playback. Keep in mind that the use of a higher bitrate for your songs (such as 256kbps and above) will result in a decrease in battery life as the iPod's hard drive has to spin up more. So, if you load up a playlist of 256-320kbps files, you're looking at a playback time of about 34-37 hours. You should get 40 hours if you use a bitrate of 192kbps and below though.
What I said about the battery takes into account them charging it up once or not at all. iPods come with a charge directly from the factory. So, even if they never ever turned it on (which is hard to believe), the battery would have had a charge in it anyway. Then, after sitting for years, the iPod's battery would have drained. So it really doesn't matter if they charged it up once or twice and then let it drain or they just let it drain directly after buying it.
A replacement battery is less than $20 for the iPod classic but, if it is really a concern, you need to take that $20 purchase into account when buying the iPod classic from the seller. In other words, if they are currently charging you $200 (this is an arbitrary number) for the iPod classic, you need to argue them down to $180 since you're going to have to spend $20 and get a replacement battery. You should be able to argue them down further since you will have to take the time to conduct a battery test, buy a new battery, and take time to install it.