You do know that if ANY pokemon is traded, it effects the Pokemon you've put up?
So offer your Haunter for something crazy like a lvl 100 Arceus and then search for people to trade with, look for Geodudes & Bidoofs, etc etc, crazy common Pokemon.
Once you conduct one of those trades, take your Haunter out of the exchange and he'll evolve.
I've tried this, can't ever find a trade partner. I look for Rattatas and Pidgeys and people are looking for lvl 10 and under Celebis. -_-