I dont think it was a limited time gun. It was given out in a quest, and the quest got removed eventually, maybe due to map updates etc. So they decided just to sell it. Think of it this way, you got the gun for cheaper. So did I...

But they never said the guns were limited time.
Plus its too late now, dont you think. Its been there now for weeks/months.
How could they know who did the original quest, to give them a golden version? Just move on.