I agree; better safe than sorry. The comment that kind of burned me was the one the dealer made about being on the jury. Hey, lowers, uppers, all the pieces cost money, and take time to get. I can't really see any reason why you wouldn't get a kit with all of the parts, but what if you wanted to build from spare pieces laying around (how many of us have "spare parts" that we swapped out on another project)?
I guess I'd look at it this way--if the guy had the upper for a month and then went and bought the lower, that might demonstrate intent. If the guy bought the upper along with his rifle in 1990, he's obviously been using it, and that may not be intent. I guess it would just depend on the circumstance. The jury comment really rubs me the wrong way, though.
Anyway, thanks for the opinions.