The dealer is correct; I don't think BATFE spends a lot of time worrying about it, but their official position is that people collect guns, not receivers.
The new barrels and stocks raise a question. If the barrels and stocks are sporter type, then the nature of the guns has changed, and they ceased to be C&R when the new parts were put on . But if the new parts are identical to the old military parts, then the nature of the guns has not changed, only parts have been replaced and they are still C&R. (Or at least that is the way I understand it.)
The rulings in that area are confusing, and I really can't blame the seller in this case for exercising caution and wanting to ship to a licensed dealer.
The question at this point is that if the OP had receivers shipped to him under his C&R license, the shipper has made an error. So what to do about it? I don't know. It might be OK just to log the receivers as if they were C&R rifles and let it go at that, but maybe a call to BATFE is in order if there is a lot of concern.