~The coding for these investments prevents people from selling the bitcoin after purchase, so investors are left with a valueless investment.
I don't understand the last sentence but taking it with what you're trying to say. It's true that these scam investments coming from tokens, coins and even NFTs, they'll just leave their investors with nothing once they're done with the project.~
What he meant is likely a type of smart contract that is designed to prevent the selling of a token, a kind of the Squid Game smart contract scam. So the smart contract is just coded with a specific address in mind, which is, it's the address of the smart contract creator itself. That specific address is the one that will only be able to sell tokens, every other else can't sell it, but they are only able to buy or move it.
I was not even aware that something like that existed, so those scammers are creating smart contracts in which no one except themselves can sell, this makes me wonder, if they have the ability to do something like that wouldn't be better for them to actually try to develop something useful with that technical skill that they have instead of using it to try to scam people? Because I cannot do something like that and yet they can which means they have some skill, but instead of using it for good they have decided to do bad things with it.