This is the intellectual property of OP and should be protected by law.
Now that things have happened, I will now give the OP some suggestions for resolution (According to general law)
1. Asked him to return the money for the sale of the script to the OP, and stop his sale.
2. OP should seek some protection of the certification of official institutions, such as
USPTO or other national patent offices, but this may cost some money, you can submit your patent application online.
3. I noticed that you have such an article in your original post:
- Price stated below is for one copy of the script, that means I intend to sell it multiple times. In case you want to be the only buyer please contact me
It means that you allow to transfer script, if the other party pays you enough money, then he will take ownership of the script and can legally sell your script. So you can ask the other party to pay you more money and get the ownership of the script.
The above is just a suggestion based on the usual laws, but I don't think the liar will follow this advice, you can try.