The ink-signed original FFL copy between licensees is required by federal regulations.
You can look it up in "Federal Firearms Regulations Reference Guide, 2000" ATF P5300.4(01-00).
That regulation only applies to transactions between licensees, so for non-licensees, they've got to comply with the Gun Control Act of 1968, which requires that only licensees may generally receive firearms in interstate commerce.
In that case, making sure the licensee is listed in the ATF FFL EZ-check, or they've been to the store before, or know the FFL personally to be a dealer, etc is usually sufficient ('reasonable man interpretation') for the non-licensee to comply with the law in that regard.
Any licensee will give you their first 3 and last 5 digits, although some will choose not to send out signed FFL copies to unlicensed persons, or to not receive firearms interstate from non-licensees at all.