Most common advice is that you have to be at least one of the sides somehow. Reddit famously did this with lots of sock puppet accounts to foster discussions and create pseudo activity.

In your case, I'd probably start by reaching out to businesses in mid-size metro markets, ones where bike couriers don't already exist, and offer to save them on shipping small packages. Build up a list of clientele and encourage them to contact you for jobs when they need to ship small ad-hoc stuff. That should give you an idea of demand. Then start posting on craigslist and facebook looking for delivery drivers, then start match-making. From there encourage the drivers you find to sign up on your platform for future work.

this is really helpful, especially the "build a list of clientele first" part. I've been so focused on the product that I havent done enough of this groundwork. The craigslist/facebook angle for finding travelers is smart, it crossed my mind but wasnt sure

Yes, we called API - Actual Person Interface. One person on one side of the marketplace that does the heavy work to build the other side, kickstart the flywheel

Agree, and for the delivery-riders side, you and some vlose people can start making the deliveries if possible.

I dont think there is really an alternative to juicing it. Frankly I would do both sides even and make the activity very visible.