I suggest you do the following:
- Each time you sell a script to someone, get his Roblox account’s ID number.
- Modify your script to work in places only with that ID by simply checking the CreatorId.
- Obfuscate the modified script and give the guy the obfuscated version. Keep the original one to yourself only and re-do the same process each time you sell a script.
Your code can no longer be sold for cheaper by the people who purchased it.