The following applies only to the US. Since 1968 you need a license granted by the federal government to sell guns (https://www.atf.gov/firearms/qa/who-can-obtain-federal-firea...). The law allowing you to carry a firearm was passed in 1791. Driver's licenses have been around since 1899. You can't sell a car to someone without one.
When thinking about this issue, I've found the following thought experiments useful:
(1) Should someone who ran a multi-million dollar illegal gun operation get life in prison, even though unlike drugs, the right to own firearms is explicitly protected by the Constitution?
(2) Should someone who ran a multi-million dollar website selling only weed in legal venues (Colorado, etc) be convicted of any crime, never-mind sentenced to life in prison, even though it is against federal law?
Personally I answer (1) as YES and (2) as NO, and place Ulbricht's conduct significantly closer to (1) than to (2).
Honestly, I can't tell if this block is because I'm conflating the context of the greater discussion (the Ulbricht trial) with the more nuanced points of timsally's comment.
Thanks!
You most certainly can. And it's completely legal. Driver's licenses have nothing to do with buying and selling vehicles. Some (all?) dealers might not do it, but there's other reasons besides legality for them to worry about.