Q: disaster

Leaks can be repaired and some leaks are frankly trivial issues in terms of difficulty of repair and cost. So, it’s only a disaster if it’s a "bad" leak where bad means "costly" to repair. As long as you didn’t pay too much for the car and it is otherwise in good shape, you might have just simply a leak to deal with. You could first ask the seller to repair the leak as a courtesy; they might do it if it’s a simple matter. As far as returning it, you first have to see if the leak is costly to fix. If it’s not costly to fix, then it’s pointless to return the car because presumably you need a car and all cars need repairs at some point so a leak that is not too costly to fix is not an unreasonable circumstance. If it’s a serious leak, no harm in asking if you can return it. To conclude, I would first see if they will help you. If not, and as far as YourMechanic, certainly you could request a certified Mechanic would then perform a leak diagnostic, identify the leak and let you know of the cost to fix it. Once the mechanic looks at the car, as long as you didn’t overpay for the car, and the leak is not costly to fix, it may very well be worth to just fix the leak and continue on your way.