First figure out which side is harder to attract. Demand side or supply side? In most cases the demand side is harder as you could easily get people/companies who want more $. This however does not hold in every market.

I am going to use the term the marketplace driver for this. Not sure if it already exists.

The market place driver will make the platform attractive for the other side.

A strategy for this could be to intially offer something of value to the marketplace driver without the need for the other side. So they get value without having to conclude a transaction.

Then eventually transition to the marketplace business model.

Another tip would be to focus. like around a geographic location, product or niche service. Then expand out from there.