I have bought and sold on Gunbroker. Any reputable dealer would have put a note on the auction, that the photos are "typical" of the guns being sold, but not necessarily the gun you will receive. When you know that going in, it keeps you from paying top-dollar for a possible beater.
If I were in your situation, would I have a problem? YES. But I would have probably handled it like you did.
Let the guy know that YOU KNOW what he did, get some cash back, and move on.
That being said, it would really bother me to buy a rail gun, and open the box to find it "naked".