I do not know much about testnet Bitcoins, but could you kindly shortly introduce it? Do you think that testnet Bitcoins will ever have a value? Also there are only 21 million testnet Bitcoins? Sorry, I have heard about the testnet Bitcoins for the first time.
Its an alternative blockchain for bitcoin, mainly used by blockchain app developpers to make experiment tests without the need to use real bitcoins. As stated in above posts, you can get some for free using one of the faucets, hen usually send them back so can be used by others who may need it. It's not for sale, and as you can see the address format is different from the ones of the real bitcoin to avoid possible scams.
You can read more here:
https://en.bitcoinwiki.org/wiki/Testnet