I would think that the first thing you might want to check is whether the light is actually correct. If the light is on, even with the key off, you clearly have a drain on the battery. Next would be to check the voltage at the battery with the key off, with the key on and the bike not running, with the bike idling, and with the engine running at 3k rpm & 5k rpm.
Ignoring warning lights based on the statements of the seller may not be the best approach.