marketplace
ˈmɑːkɪtpleɪs/
noun
Definition – A place where buyers and sellers connect and conduct transactions.

The definition holds true in the online setting as well. For example, GrubHub is a marketplace where diners order food from the local restaurants. Similarly, Airbnb is a marketplace, where the guests select a short-term accommodation for themselves and the owners rent out their properties or spare rooms to guests. The marketplace website acts as a middleman taking care of the payments which makes it convenient for both parties.

SlipSeat is a marketplace too, but instead of buying dinner or a renting a cool apartment for the weekend, carriers shop for drivers for short-term driving gigs.  The drivers themselves are the sellers.  SlipSeat is the middleman, providing the framework and processing payments.