Also you don't know how shot the battery is because it could have been lightly used or heavily used.
That's what I am afraid of.
There is a huge difference between second hand, and refurbished. Refurbished replaces parts that need to be replaced or repaired. The device is essentially, like new.
If you’re looking at iPads, look at the Apple Store online. Refurbished units for many different devices are available. AppleCare available also.
Repairing a device doesn't equal replacing a weak battery. The weak battery is still working. Maybe it only lasts 2 hours on full charge but it still works 100%. A refurbisher is not going to replace that battery unless it's dead and your warranty is not going to cover "battery is not lasting long enough to my liking".