A lot of gun shops, as I understand it, won't take a return on a gun that's been out their door. But it's not the store who will probably fix your gun... it'll be the manufacturer. I've had to send a couple of guns back to be tweaked... to Winchester and Colt for different reasons. They were under warrantee and the manufacturers did the work for no charge. In the case of the magazine tab, you're only talking about replacing one part... right? If Marlin's the stand-up company it's been said to be, they do the same. Or, you can get online at Numrich (e-gunparts) and probably get that part and replace it yourself for little money and less waiting time.
I don't know whether I'd shop at the shop in question or not. It may or may not be a problem. Did they say they could send it back or did they offer anything at all?