Most gun shops I've visited have had illegal items for sale at one time or another to non-LEOs (that these items are "illegal" to begin with is the real crime - but that's neither here nor there). I believe all examples were due to ignorance of the law. Here are just two of probably 20 personal examples I could list:
1) Buffalo Gun Center had 15-round Beretta 92 Billennium magazines for sale last year in the non-LEO section (the Billennium pistol hit shelves in 2001, so magazines made for it obviously are not pre-ban). Yes, I bought two myself - and lost $86 when I ditched them after realizing my blunder (I should have realized what the word "Billennium" meant - duh!). They've since stopped selling those mags to non-LEOs.
2) I recently purchased a used Colt 6400C from Jackson Guns & Ammo (I am not an LEO). This gun was manufactured in 2002 and originally featured a permanent muzzle brake, a pinned M4 stock, and no bayonet lug. Holding the rifle at the counter, I noticed that the previous owner had swapped the pinned Colt stock for a non-pinned Tapco unit, making the weapon illegal in NY. I decided to purchase the rifle anyway, but informed Kordell (the owner) that the stock was illegal and that I'd like it removed before purchasing the gun. He removed the stock and ordered me a fixed A2 unit. I left with the (fully-disassembled) rifle that day and simply picked up the stock a few weeks later.