There's no good way to disguise something like this. Knowingly selling a car that has a major leak like this and not disclosing the problem or attempting to "band-aid" it until the sale is complete possibly constitutes fraud, and is at least a bit amoral.

I'd think it would be best to figure out what's leaking - it could be something as simple as a rear seal or a pan gasket - cheap and easy to repair. When you find out what's leaking, get a quote from a reputable mechanic to fix the problem and be ready to allow that as a discount to your asking price - you may find that being forthright about the shortcomings of the car makes it easier to sell, not harder.