The previous owner installed a third party battery in the laptop since the manufacturer is listed as "TOP". Unfortunately the quality of third party Lithium-ion Batteries is extremely poor even when purchased from a respected vendor such as OWC or iFixIt. Your best option is to replace the battery with one from OWC, but it is a difficult & risky process since the battery is glued to the underside of the palmrest & trackpad. Be very careful not to physically damage the battery since it can be easily bent and punctured which may result in a fire. OWC has instructional videos on their product page showing how to replace the battery (it always looks easier than it is).
https://eshop.macsales.com/shop/Apple/Laptop/Batteries
The Apple Diagnostics will most likely report an error with the battery just because it is a third party battery.
Also, I hope you performed a clean install of macOS by first erasing the whole physical drive before reinstalling macOS so that you have a factory fresh OS and file system. You never know what nasty surprises may have been left behind. Performing a clean install will instantly tell you whether a firmware lock has been enabled and perhaps even whether the laptop is still being managed by a previous owner. Here is what the previous owner should have done before selling the Mac:
What to do before you sell, give away, or trade in your Mac - Apple Support
Even if the previous owner performed a clean install, I still highly recommend users do their own clean install just to be sure it was done and done correctly.
