Testnet coin is used for testing purposes as it should be. You wrote testnet3 and you would know that there are 3 generations of bitcoin testnet. If bitcoin testnet 3 generation has value, bitcoin developers will create bitcoin testnet 4 to force the bitcoin testnet3 to fall back to zero value.

It is a fraud if you are invited to trade bitcoin testnet with real value.

I know bitcoiners have to spend money for bitcoin mining rigs to mine bitcoin testnet, but official trade it is fraud attempts.