Tom is right on with criminal liability. Why does everyone recommend doing things based upon an ability to prove who yourself innocent. Last I checked the government was required to prove you guilty.
As for civil liabilty:
In order to recover for negligence the plaintiff must prove duty, breach, cause, and harm.
What duty do you have to prevent a gun you sell from subsequently, and unbeknownst to you, ending up in a third party's hands? Not only do you not have a duty it would be impossible for you to do. If you don't have a duty you can't be liable.
Further, the acts of the person you sold it to would likely be a superseding/intervening cause that would break the chain of causation from you.
Now if you live in a state that has universal background checks and you sell a gun without doing one you may be both criminally and civilly liable. But even in this case civil liability depends on the ability to prove causation.