The grounds are not there for such a lawsuit against the web site.
It is possible that should a member of the website go against policy and federal law, that the seller and/or buyer could be sued.
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Life Member NRA
Life Member NAHC
Former President of the ECPT (Eifel Combat Pistol Team)