Your assertion that were large selloffs of companies like AA was interesting, so I did a little research.

In the case of American Airlines, Yahoo's stock quotation system (this URL may or may not produce the same chart I used) showed that the stock had been declining gradually since June 2001 ($43.75 on 5 June), and on 4 September the stock had plateaued at $37.50, then dropped like a stone to $34.75 on 10 September. I think the stock market was closed for a while; the next datum is $32.40 on 17 September.

Well, "drop like a stone" may be more an artifact of the scale on the chart, since the actual loss in value was around 10%. I wouldn't call it a "massive selloff", but then, if there was something sneaky going on, only a small number of people would have known, and their selling would have only a small impact. To continue the suppositions, these hypothetical insiders would have been the kind of people to own large blocks of stock, so perhaps an investigation ought to look at who sold large blocks of stock over that one week period. Also, you'd need to compare the performance of the suspect stocks against the market overall, and airline stocks versus airline stocks, as well as check the news to see if something like an increase in aviation fuel prices was a factor. You have to judge the drop in price in its proper context.

Pretty murky situation. I wouldn't call it a promising line of investigation for people looking for a conspiracy around 9/11. If there's anything to it, you're after the hardest people to catch, people with the connections and expertise to hide stock ownership and launder money.