If they are a US citizen, they must scan both driver's license/state ID/passport/military ID and social security card. If not a US citizen, they must scan their home country's passport and if they come from a country that requires one, their visa.
Also, require the purchaser to type in (not check a box, but actually type the words and disable the browser's paste function) "I swear under penalty of perjury that I will not transfer this ticket to anyone else and that I will do everything I am physically capable of to not let anyone do so on my behalf".