+1 for getting a refund, or at least a partial refund to cover the cost of the repair/frame replacement.
If you end up keeping it, hit up Tom Bostic of Tommy Built Tactical to see how much he would charge to repair that. He's fixed far worse, and it will look near perfect if not perfect when finished.
Honestly, it's my least favorite HK to shoot, and I would have gotten rid of it a long time ago, if it weren't for the stainless slide. The stainless finish HK has on these guns is just plain beautiful, and it's the only reason why I still have it.
Check all the typical used gun sites (Gunbroker, Armslist, Guns America, here, etc.) regularly, and be patient. And perhaps find a non stainless as a shooter gun in the mean time, since it seems like you might want the stainless finish to stay shiny.
Get a REFUND ASAP! I wouldnt accept a damaged receiver, find a better one next time , best to find one locally so you can inspect it.
Full refund. No exceptions. You have a good case. But be prepared to shell out more dollar$ for a .45 stainless. Sellers want a premium.
I would file a GB complaint and demand a full refund including shipping and all other fees, that's totally unacceptable, as the seller was obviously trying to hide the damage, even worse they didn't disclose it...SHAME on them!
I would return for refund without hesitation!
a gun show vendor tried the same thing with me with a West German P38 , the slide was cracked, but he tried to hide it with a price sticker, then when I pointed it out he played dumb like he didnt know anything about it, you have to be extra careful buying used at shows, thats where people try to get rid of messed up guns and dont tell you about it.