Can I sell a Sig 716 DMR to someone in New Jersey?

Well I would contact his FFL. If that guy says this can be done and is willing to do it, then do it. The guy would be out of his mind to put his license and his OWN ass on the line if something like this wasn't feasible. The ATF would be all over him.
 
The above two posters got it right. If the FFL is willing to do the work and put his license on the line its perfectly legal for you to ship to them.

Flash hider would be a no-go, but he can easily silver solder on a muzzle break. The UBR would need to be pinned (as you said) as would the 15+ round magazine. Once all that is done by the FFL he can legally transfer it to an NJ resident.
 
You are actually selling to a dealer and, at least IMHO, if that dealer emails you a copy of his FFL then he is saying it is OK for him to accept your rifle in its current state of manufacture. What he does with it determines legality or not. A quick confirmation over the phone with the FFL holder would help ease one's mind though.
 
You are actually selling to a dealer and, at least IMHO, if that dealer emails you a copy of his FFL then he is saying it is OK for him to accept your rifle in its current state of manufacture. What he does with it determines legality or not. A quick confirmation over the phone with the FFL holder would help ease one's mind though.

I've got a better idea. Just tell the guy in New Jersey to GTFO of the Garden state as it is a royal hell hole for gun owners!