They are worthless, but useful. They are useful because they are worthless. If you will add value to them, they will be useless, therefore worthless.
Exactly.
DO NOT SELL TESTNET COINS. You'll just piss off the core developers, because we'll have to waste time re-launching a new testnet to reset the chain and make the coins worthless again.
There a Fight Club "First Rule..." joke in there somewhere....