Looking to sell a complete handgun upper to someone in Long Island City NY.
I was wondering if this is legal to do or do I have to send it to a FFL?

I'm not aware of any law that says you can't, although the buyer could be in trouble depending on what he does with it. Long Island City is part of NYC, which is significantly more restrictive than the rest of the state. Can you? yep. Should you? Nope. It's worth mentioning that they'd need to amend their pistol license concerning this upper, so if they're willing to have you send it to an FFL, I'd suggest doing that. It'll make it easier for them to take the receipt down to 1 Penn Plaza for the amendment.

I don't know if this is a defining criteria, but curious, is it serialized?

I had to replace a Glock frame once and I recall
that it wasn't exactly staraight forward.  Ultimately they decided that they only cared about the serial tag on the lower frame, which I think is correct.

No, I decided not to sell it to the person. Just to be on the safe side

No problem at all: that upper is nothing but parts, it is incumbent upon the purchaser to insure he's in compliance with any State, Federal, or local Laws. Cavet upon the buyer: having such with a rifle lower could be construed as "constructive intent" but unless you've invited the man into your life, then its not likely an issue.

The Frame or receiver portion that contains the serial number and well as make, model, etc. IS "the gun" and therefore regulated.
Granted the know-it-all's that run NYC have differing views, but the fact is that upper, and in the case mentioned about a Glock (which is serial numbered on the slide and barrel as well as the Frame) those numbers only identify the original Pistol. Again, parts is parts.  

It shouldn't be illegal on your end...

When the slide becomes more than "just a part" in NYC is when it helps to convert the caliber of the pistol.

Thanks to our nonsense laws, you can switch calibers by changing just the barrel, but if a separate slide is needed, then it has to be registered as a new handgun. At least that's what I've been told by 1PP.