What I meant is the gun(s) that the undercover ATF agent is selling has these characteristics, then busts you for buying/possessing a firearm with a removed serial number or that was imported illegally. So (again very unlikely, just an example) the agent had made the gun illegal in these fashions and once you buy it he charges you.
Again, this is based on the person buying the gun reasonably KNOWING that it's illegal.
For example, a removed serial number should be obvious. (By the way, a removed serial number in an of itself does not constitute making the gun illegal, so long as there is appropriate documentation of this. I once bought a 1911 from a friend with a partially removed serial number. It had been stolen and subsequently recovered by the police in this condition. He was able to identify the gun through other unique markings and get it back from the police. I bought it from him and got a copy of the police report documenting the damaged serial number marking. Later sold it to someone else, also with that documentation.)
An illegally imported gun may be another matter, however. If it's a gun that's flat out not legal to import into the United States, then that should be something that's obvious to anybody who may be interested. However, if it's an otherwise perfectly legal gun to own, how is one to know that it was illegally imported?
The crux with buying such illegal guns as you mention by example is that the buyer has to be capable of reasonably determining that it's, in fact, an illegal gun. Otherwise what ACTUALLY happened is the SELLER fraudulently SOLD an illegal gun under the guise of it being legal. And THAT is a felony. What the buyer did was purchase something that was being PRESENTED as a legal product.
Imagine how the following particular scenario might go over in a court of law:
Prosecuter: This man is charged with illegally purchasing a fully automatic Thompson Submachine Gun from undercover ATF agents.
Defense: Your honor, here is the bill of sale for the firearm in question. It shows in writing that my client was sold a semi-automatic Thompson Submachine Gun, S/N XXXXXXXX. There are no external signs or other evidence present on the firearm which shows that it was ever tampered with or otherwise visibly engineered to be a fully automatic weapon. In addition, the security video recordings of the sales transaction that took place during this supposed undercover ATF operation clearly shows that the firearm was being advertised and marketed as a semi-automatic firearm. My client thus purchased a firearm that was presented to him as a semi-automatic firearm, which is perfectly legal in accordance with federal law. This means that the ATF agents setting up this sale illegally sold my client a fully automatic firearm while presenting the merchandise as a 100% legal semi-automatic gun.
Judge: Defense and Prosecution come to my chambers.