Always charge the side which bigger so in this case you take the money from demand side. Here you service is actually finding for the demand some supplier.

We had it well developed over the Socialism in Eastern Europe - you woud pay just to get in the que for some product

You can charge demandin 3 ways (at least):
1. for access to the marketplace / offers from supply
2. % from every transaction
3. Create price plan for speedy fullfilment of the orders and charge basically for being first in theque. Could be auction based pricing

I'm both a startup advisor and parent, so I can relate to this. I'd be interested to talk to you about it.