You can test the Stripe checkout flow on your marketplace by using development client IDs, re-direct URL, publishable and secret keys, before bringing your marketplace to live when you are ready.
Essentially, you just have to follow the steps listed under Setting up your payment gateway (Stripe) found in this section of the Help Centre, with the exception of copying and pasting the Test mode client ID instead of the Live mode client ID in Step 10.
Once you are ready to move on to live transactions, do remember to input the Live mode client ID, production re-direct URL, publishable and secret keys before launching your marketplace. We recommend that you skip this section if you are not confident of trying the development mode.