I am buying a used Honda Accord 2004 and yesterday I brought it to the garage and the mechanic said that everything is good except that the coolant is empty. Yet according to the owners, the engine does not give any overheating warnings. The AC also works fine and the radiator fluid is full. What could be wrong with the car and is it worth to buy it?
It's impossible to say without further investigation.
Assuming "the coolant is empty" means there's no coolant visible in the little transparent tank, it might mean the coolant level is a little low, and filling the little transparent tank might be all that's necessary. It also might mean that there's something wrong with the radiator or a radiator hose that causes coolant to leak away, or something else is wrong that causes the car's coolant to leak away or be consumed in one of the cylinders. These things might be easy to fix (change a bad radiator hose), or harder to fix (replace the water pump or the head gasket). and adding more coolant will only help for a very short time.
There are two human-related issues as well: 1) I'm not impressed with the mechanic's report: he cannot know that "everything is good" unless the cooling system is full of coolant and operating correctly; and 2) It's entirely possible that the current owner, wanting to sell the vehicle, isn't telling you the truth.
If I were there, I'd want to add coolant and run the engine and see what happens. I wouldn't buy the car in this unknown condition.