If/when an out of state resident moves into NJ with legally obtained firearms that are NJ state compliant, I am of the understanding that the individual ONLY needs to get a FPID card if they intend to purchase additional firearms or handgun ammunition.

Say the person does not apply for a FPID and decides they want to sell one of their firearms (long gun) to a NJ resident with proper DL/FPID. The NJ COE form asks for both transferor and receiver's FPID card number so if in the above situation would a private sale with a COE even be possible or would the seller also need to have FPID or would this have to go through an FFL?

No. Only buyer needs FID

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^^^THIS. And seller leaves "Transferor’s Firearms I.D. Card Number:" blank or N/A.  

^^^THIS. And seller leaves "Transferor’s Firearms I.D. Card Number:" blank or N/A.  

Correct.

No one sees COE anyways except buyer and seller.

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Here's your third agreement on the subject. Now go buy it and enjoy it.  

4 hours ago, flank said:

If/when an out of state resident moves into NJ with legally obtained firearms that are NJ state compliant, I am of the understanding that the individual ONLY needs to get a FPID card if they intend to purchase additional firearms or handgun ammunition.

Say the person does not apply for a FPID and decides they want to sell one of their firearms (long gun) to a NJ resident with proper DL/FPID. The NJ COE form asks for both transferor and receiver's FPID card number so if in the above situation would a private sale with a COE even be possible or would the seller also need to have FPID or would this have to go through an FFL?

A "Seller" could be a legally blind and deaf octogenarian who only owns firearms through them being willed to him/her.  Even decades ago.  These family heirlooms may not even have serial numbers (in some cases).  A "Seller" could also be a PROHIBITED PERSON who was willed the firearms and has 6 months to sell them and notify his/her CLEO that they're sold (in most cases the Prohibited Person books these into a retail FFL for consignment).  The "Seller" could be an Executor or Executrix of an estate named as such in a decedent's (Gun Owner's) will, or an appointed lawyer w/ power of attorney acting on behalf of an octogenarian Executrix/Executor that permanently resides in a Rest Home.  None of the firearms need be registered to the "Seller" or require any paper to prove ownership.  Some of them may even be War trophies brought back from WW2, Korea or Nam, and some of those may even be stamped, "US Government Property" (1911's for example).

It's all LEGAL!

So do as TBill suggested & enjoy your purchase