Are they trading you a Haunter or a Gengar? You mention both in your post.
I'd honestly just keep the shinies unless they were spares or the trade was a guaranteed lucky, but I'm a collector so take that how you will.
Assuming you're best friends and the other person is missing one that's a 40k dust cost minimum. While still destructively fast, Shadow Claw Gengar demand has fallen off hard due to Lick being more widely available and performing roughly the same.
If the trade actually *is* for a Haunter and not Gengar then forget it. Haunter knowing Shadow Claw will not be retained when you evolve it.