I wouldn't count on two new rifles having interchangeable barrels, they might, might not.
I've never bought a bad frame from anyone, have bought well over a dozen, I suppose it depends on who you're dealing with tho, if that happened here, I doubt the seller would still be here. I wouldn't let an isolated instance like that sour you on buying frames, I'd be more concerned about buying an SB1 frame that is said to be an SB2, but with the serial number, that can be verified with a phone call to H&R.

Tim