I usually give them the local sheriff's home address and number or the state attorney general office info if I feel like replying.
The bad thing is, they would not be taking the time to manually enter your number and text if someone wasn't falling for this crap.
I like to contact them and ask if "You still have the POS?" They often respond with something like: "Yes, the POS is in good shape!"
I will then string the idiot along for two weeks until I tell the scumbag how dumb he/she is for trying to sell a "piece of ****."