I've gotten by many retailers (while I was 16-17, now I am 17), most of above listed. Although I did conduct myself fairly maturely (not to gloat) so don't be to sure if you know a retailer or not. Although its not like I'm angry with them, its a hard business with limited customers as it is.
ASC is the hardest place to purchase underage for me. Although I've been able to come aware of sales of guns through different ways other than the classifieds such as; signatures (which is now banned), Pbase accounts (looking at one for accesories and see pics of guns that are labeled for sale), and MSN names (which is just plain out of the control of admins obviously).
Although I've never inquired any guns on here a good way to rid out two of the loopholes is to force buyers to post something in the For Sale thread that contains the product the customer is buying to be sure that they are age verified. Or just add some kind of feature (mabye something down near the AIM/PM/MSN/webpage things are under the trader rating on the side panel of your posts) to show an age verified member.