When members in your marketplace start a transaction, they'll have to follow a few steps until the transaction is fully over. You can learn more about the different flows at:
Mark as completed by buyers or admins
One important step is to mark the transaction as completed. When this is done, money is transferred to the seller's bank account (if Stripe was the payment gateway), and both users can then review each other.
This is something buyers should do, from the transaction thread in their Inbox when the service or product has been delivered.
This is also an action admins can do to move things forward on behalf of users:
log in to your marketplace
go to your Admin panel
go the the "Transactions & Reviews / Manage transactions" page
click "View details" for the transaction you want to update
from the Transaction view, review the current situation and use the action buttons to move things forward, if available