https://sgp.fas.org/crs/misc/R43769.pdf -- Accomplices, Aiding and Abetting, and the Like: An Overview of 18 U.S.C. § 2
Conviction under Section 2(a) requires that a defendant embrace the crime of another and consciously do something to contribute to its success. Although its elements are variously described, it is often said that, “[i]n order to aid and abet another to commit a crime it is necessary [1] that a defendant in some sort associate himself with the venture, [2] that he participate in it as in something that he wishes to bring about, [3]that he seek by his action to make it succeed,” and [4] that someone commits the offense. Satisfying only one of these elements is not enough. Thus, presence at the commission of a crime or close association with the perpetrator does not constitute aiding and abetting, without more. Yet, a defendant’s level of participation may be relatively minimal and need not advance every element of the crime. As for seeking to make it succeed, the defendant must intend the commission of the underlying offense, and that intent requires that he be aware beforehand of the scope of the offense in order to permit him to disassociate himself. Thus, the defendant must know that the offense is afoot before it occurs if he is to be convicted of aiding and abetting.
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So... it depends? If someone walks into your bookstore and says, "Hey, I want to cook up some meth" and you say "I know a great chemistry book but i'll have to special order it for you" that starts to look like association to me. But buying a book off the shelf, ehh, that's why you have books on the shelf. Guns are used in crimes. Cars are used in crimes. How often do you hear about car salesmen being arrested for aiding a bank robbery? Or gun salesman for a mass shooting?