Have a 2004 Pontiac Grand Prix with good body that expelled freeze plugs, on replacement it looks like there are bigger problems; Is there a market for this car?

It's 10 years old. The cost of a crate replacement engine excides the value of the car. However, if you can get a used engine and can find someone to install it, it might be worth keeping. Only you can decide that.

I wouldn't spend another nickel on it . Unless you are equipped and inclined to do the engine swap yourself the cost of removing and replacing your engine with a different one is not likely worth it, especially if it i just a basic GP. Advertise it as a mechanic's special and try to get a few dollars for it.