Short answer: Choose Supabase for a server-side marketplace whose documents depend on relational ownership data; choose Firebase Storage for mobile-first upload feedback, and S3 for the broadest large-file operating model.

For a marketplace storing private user documents, the storage engine should be chosen around the download path, not the upload button. Supabase is usually the simpler fit when document records, ownership, and relational checks live beside the object. Firebase Storage is the stronger fit when a mobile client needs rich, real-time upload monitoring. Amazon S3 is the baseline when large-file throughput, ecosystem depth, and portability matter more than an integrated application experience.

The practical design is the same in all three cases: keep the bucket private, authorize a document through application data, and issue a short-lived download link only after that check. An expiring link is a delivery mechanism, not an access-control policy.…