It depends on how much liquid and where. What often happens is that if the console sits wet for too long corrosion builds up on the contacts in the ZIF connectors, which means that even if the screens and other parts are undamaged, the console won't recognize that they are connected. If this happens it can be very difficult or impossible to repair.
Yet on other occasions liquid damage is easy to fix with a thorough cleaning of the motherboard with isopropyl alcohol. It all depends on the way in which the liquid spread around, so it's really just a matter of luck. There's not really any way to tell without opening it up, but if you can see liquid marks on the screen itself, I'd suggest staying away from the deal unless it's very cheap, since that would suggest it was a large amount of water.