Re: Q's Regarding Firearm Sales.
When I do online/Internet/non-FTF sales of any kind, I maintain a copy of the FFL for my records and I keep them indefinitely, along with other pertinent info on the sale like a description of the firearm, pics, serial numbers, comms with the buyer, etc. In the "digital age" it is easy to keep a scanned electronic copy (PDF or other format) if "space" or keeping paper floating around is your concern. ATF specifically allows faxed copy, electronic copy, etc. of FFL paperwork for transfer purposes, so I see no reason why keeping an electronic copy is insufficient. As for how long...I have only been engaged in buying/selling firearms online for around 9yrs or so and I still have the paperwork from each transaction. YMMV, but I honestly can't point you to a provision in federal law that says you must maintain for x-number of years.
As for FTF sales...I <span style="font-weight: bold"><span style="text-decoration: underline">ALWAYS</span></span> get a copy of the person's DL and, if they have one, their concealed carry permit (CCP) and I won't do business with people that refuse to provide that information. This does several things, including verification of age, address, and other vital stats and if they have a CCP, it shows at the time it was issued that they passed the BG/Criminal Rec. check. Also, living on the border with two other states, I have had several people want to buy handguns from me when they were not residents of my state as indicated by their DL. I won't do that for obvious reasons. Again, I keep copies of that documentation. As for your "Bill of Sale/Transfer of Ownership" document...I don't know what's in your doc, nor do I necessarily need to know, but I will tell you that it is meaningless in terms of the "legalities" of a transfer or to shield you from potential civil (and possibly criminal) liability if the person you sell the firearm to turns around and does something stupid (to himself or to others). Now, I'm not saying you are going to get hit in the bank account or go to jail...but that doesn't stop an angry person from suing you in civil court or stop a zealous district attorney from pursuing criminal negligence, etc. charges against you.
If you are that concerned about liability, then for future FTF purchases, involve a local FFL dealer who will probably handle a transfer for $25 or so. It is money well spent if you feel better or have piece of mind about the future liability. If the buyer completes a 4473, gets cleared for the transfer/purchase by our friends at the Bureau, then that pretty well takes you off the hook if he turns out to be a raving lunatic or an idiot who injures himself or others with the firearm.