Dead crank
 

I a 2001 Ford F-150 5.4 triton. I bought it and doesn't run it was a good deal so I jumped on it. The guy that sold it to me said he was driving it and oil leaked out of it he drove it for a mile and parked it and could never get it to turn over again. He said a mechanic looked at it and said engine was seized. They checked the oil level and was missing oil they added two quarts. So I brought it home, I put a breaker bar to the crankshaft and it turns with no issue don't hear any noises. Now I look at the transmission oil is leaking from transmission and starter is full of oil. When you turn the key you get nothing as if battery was dead. Can the issue be in the transmission? And if I swap out starter and try to crank can I cause more damage?

I a 2001 Ford F-150 5.4 triton. I bought it and doesn't run it was a good deal so I jumped on it. The guy that sold it to me said he was driving it and oil leaked out of it he drove it for a mile and parked it and could never get it to turn over again. He said a mechanic looked at it and said engine was seized. They checked the oil level and was missing oil they added two quarts. So I brought it home, I put a breaker bar to the crankshaft and it turns with no issue don't hear any noises. Now I look at the transmission oil is leaking from transmission and starter is full of oil. When you turn the key you get nothing as if battery was dead. Can the issue be in the transmission? And if I swap out starter and try to crank can I cause more damage?

What if you removed the starter and tried to determine if it is full of trans fluid or engine oil? If trans fluid, you may have lucked out regarding an engine replacement which is a heck of a lot more than a trans replace.
If the starter is full of oil, whichever type, it can't make contact with the brushes and armature so you would get nothing. Don't want to provide false encouragement, but don't assume the worst until you check. You could be pleasantly surprised. Might have blown a trans converter seal and lost the fluid and it could have quit moving and not start because the starter was full of lube.
If the engine is cooked cranking it over a few times to see if it is seized can't do any more damage than might have already been done.