Sounds to me that the mower was run without oil. After it seized the user put in fresh oil to hide their mistake. Sometimes these units can be saved, but your odds are 50/50. If you want to give it a try, this is what I would do. You really have nothing to lose.
1. Remove the spark plug.
2. Tip the mower on its side, carb/air filter up.
3. With gloves on try to turn the blade backwards, meaning you are pushing again the sharp edge.
4. If this doesn’t move it, take a block of wood against the cutting edge and give it a few good whacks with the biggest hammer you have. If it is not seized too bad it should free up.
5. If you can’t get it to move you are out of luck, junk.
6. If you free it up and it moves freely, tip the mower back up and pull the recoil a dozen times, no plug.
7. Install the spark plug and see if it starts.