Platforms and marketplaces are both cross-side network-effect businesses (Network Effect Taxonomy: Direct, Cross-Side, and Data), but they bootstrap differently. A marketplace needs supply and demand liquidity. A platform needs a 'killer app' of its own first — enough internal scale and maturity that third-party developers have a reason to build on top of it (the model B2B SaaS platforms like Salesforce follow).
Casey shut down a premature platform push at Eventbrite because the core product wasn't yet mature enough to support outside developers profitably — there wasn't enough of a 'killer app' to attract them.
Integration-first rollout as the intermediate step. Before opening a full third-party developer platform, launch integrations with popular tools your customers already use (Eventbrite + MailChimp is the example). This proves there's real appetite for extending the product and builds the connective tissue a platform will eventually need, without requiring you to court and support outside developers building net-new businesses on top of you.
Apply: don't attempt a platform/developer-ecosystem strategy before your own core product is mature and well-served internally. Prove integration PMF with existing tools first.