The OP said he bought it online and asked if the vendor could ship it directly to a NY gunsmith. Whether the vendor was a unlicensed person or a licensed dealer, the receiver can be shipped directly to a gunsmith (assuming the gunsmith is properly licensed).

The buyer (OP) definitely needs to coordinate that shipment between the vendor and the NY gunsmith.

When the NY gunsmith receives the receiver, he is now the owner of the receiver and it gets logged into his books as an acquisition. Since he should have an agreement in place with the actual buyer (OP), he can do whatever work is needed/wanted to the receiver and then, again with coordination with the buyer (OP), ship it to the buyers FFL in NJ.

The NY gunsmith would show the disposition of the receiver as going to XYZ FFL in NJ to be transferred to the OP. The original buyer (OP) again should have an agreement with the NJ FFL to receive the modified/gunsmithed receiver to transfer it to him. Once the NJ FFL receives the receiver and logs it into his books, he notifies the OP. The OP then goes to his NJ FFL and does the required federal and NJ paperwork to get the receiver he bought.

The only wrinkles to this whole thing is if the receiver were illegal in either NY or NJ (but if it is a Remington 700 action it should be ok in both states) OR if the OP is under 21. Federal law says that receivers cannot be sold/transferred by a FFL to someone who is under 21. (see instructions for question 16 (Type of Firearm) on the ATF Form 4473).