This just happened to me a few weeks ago with a supposedly brand new M4 H2O. My local dealer did the right thing and issued me a refund without a hassle and sent it back to his distributor. He did the right thing because he knew he tried to screw me. He knew that gun was gouged up and used because he had assembled and inspected it before putting it back in the box. He should have said something right then and there. If I dwell on it, I start to boil that he even thought about trying that sh**.
Being the trusting dumbass that I can be, like you, I just gave it a brief inspection and brought it home. On my way out the door, I even said, "I'm not going to bother putting it together here because I trust you." What a moron.
I have a hunch that my dealer did the right thing because I live less than a mile from him. This isn't meant as bravado or machoism - just factual - but it also doesn't hurt that while I'm past middle age, I still have a rugged, muscular build like a guy half my age, and with my shaved head, busted nose, and a few other bumps and bruises from my misspent youth, I don't exactly look like a real friendly guy - even though I am.
Well, most days. LOL
Anyway, the next day, I bought an in-store M1014 from another local dealer that I will be picking up on Saturday. I've had great success with on-line, sight unseen purchases in the past, but never again. The world is changing. So little honor these days.