When I purchased my first AR, the parts were supposed to be from such a source. When pressed for the name of the manufacturer, I was told "FN made the upper". After much searching, I found the correct person to talk with at FN. He sighed and told me that they field tons of calls about this and assured me that no legitimate source exists for resale of FN uppers. He also expressed much of what has been said in this thread. I asked him if I could pass the info, including his name and extension along to others. He said he thought it would be helpful to him and others. I have since (it has been a long time ago) misplaced the info...
The information came from FN, they were my only source, so, if he was incorrect then my post is also.
After getting in touch with suppliers that advertised this misinformation about their products (most of them said "FN manufacture" when pressed for a source), most didn't care that this was incorrect. (With the exception of this "advertising inaccuracy", I have otherwise been more than satisfied with these suppliers.)
You could check the proof mark (if it exists) on the upper to find out who made it. Mine was a "splintered 'A'", therefore, it was made by Oly.
Hope this helps,
-JmE-