Hey Mike,
Interesting. You know I am a lawyer too...
Here is the thing about that, at least in Florida. You don't have any liability for what happens to a gun as long as you sell it to a Florida resident. You have no responsibility other than to ensure they are a Florida resident and an adult, which you can do by seeing a drivers license.
Especially if you bought the gun new, the paper trail is only to you. I'd rather have a paper trail to show I sold it legitimately rather than have them just have to take my word for it.
Ideally, you get the bill of sale and you don't give the buyer a copy. Then the bill of sale is defensive only, and it will never come up unless you want it to.
To be completely honest with you, I much prefer to avoid the whole issue and only sell to dealers, or through dealers. Yes, I take a hit on "profitability," but then I have no legal, ethical, or moral responsibility for whoever takes possession of the gun next. That's just me -- and I'm not saying anyone else has to do that.
-Duane