I don't know if anybody has properly gone into switch diagnostics and failure modes yet (few people do for consoles). To that end I doubt anybody can say with some kind of percentage certainty that yeah it is usually this simple fix that will total less than a new device to fix all in, or alternatively say no it is likely a fault you are not going to want to fix.
Assuming it is sold as is that means what can you recover from it, and how easy it might be to sell such a thing (all well and good having replacement batteries but if they can't be readily swapped and have not had long enough to start kicking the bucket outside warranty/be for a device with hacks that might screw with warranty then yeah).
How much do you value the £100 odd? If it is going to be your entertainment budget for a while then that is one thing, if it is the sort of thing you blow on a random Friday night then that is a different matter.