> Every marketplace founder says "focus on one side

Unless you already have demand lined up, I would solve the supply side of the problem first. People who either travel routinely or are planning to, or just a lot of people from a specific, large community in order to target your recruiting campaign.

I would also spend a lot of time figuring out the user journey for both the supply and demand side. See if you can reduce the work someone on the supply side has to do to sign up. You might have to run this like a modeling agency in the beginning until there is sufficient demand to force suppliers to do more work.

Constraining to one route (SFO -> Honolulu) is how DHL got started.