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...find someone who does electrical diagnosis (like cars or other electronical stuff), and pay that person to run down the issue. I used to do automotive electrical, and you can spend a lot of money replacing stuff that is not broke, if you don't have some basic diagnostic tools and some knowledge of how your thing works. Any guesses anyone gives you here on the internet will just be chasing squirrels.
Stuff like putting in a battery you know is good and fully charged is something you can do, but if you need to buy a battery to do it, it's not a cost effective solution. Unless you do need a new battery.
Why would you buy an electric bike you know from your discussion with the seller is busted, unless you know how to fix stuff like that yourself ?