If a dealer or manufacturer knowingly sells weapons to a criminal that is not allowed to possess a firearm they can be sued.
If the dealer or manufacturer has sold weapons by legal means making it a legal purchase they can not.
Some additional info:
https://www.govtrack.us/congress/bills/109/s397/text
(5) QUALIFIED CIVIL LIABILITY ACTION-
(A) IN GENERAL- The term `qualified civil liability action' means a civil action or proceeding or an administrative proceeding brought by any person against a manufacturer or seller of a qualified product, or a trade association, for damages, punitive damages, injunctive or declaratory relief, abatement, restitution, fines, or penalties, or other relief, resulting from the criminal or unlawful misuse of a qualified product by the person or a third party, but shall not include--
(i) an action brought against a transferor convicted under section 924(h) of title 18, United States Code, or a comparable or identical State felony law, by a party directly harmed by the conduct of which the transferee is so convicted;
(ii) an action brought against a seller for negligent entrustment or negligence per se;
(iii) an action in which a manufacturer or seller of a qualified product knowingly violated a State or Federal statute applicable to the sale or marketing of the product, and the violation was a proximate cause of the harm for which relief is sought, including--
(I) any case in which the manufacturer or seller knowingly made any false entry in, or failed to make appropriate entry in, any record required to be kept under Federal or State law with respect to the qualified product, or aided, abetted, or conspired with any person in making any false or fictitious oral or written statement with respect to any fact material to the lawfulness of the sale or other disposition of a qualified product; or
(II) any case in which the manufacturer or seller aided, abetted, or conspired with any other person to sell or otherwise dispose of a qualified product, knowing, or having reasonable cause to believe, that the actual buyer of the qualified product was prohibited from possessing or receiving a firearm or ammunition under subsection (g) or (n) of section 922 of title 18, United States Code;
ETA Here's section 922 and 924 for additional reference.
https://www.law.cornell.edu/uscode/text/18/922
https://www.law.cornell.edu/uscode/text/18/924
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