3rd Party Platform Unique Identifier for Joins vs Internal Identifiers
To be safe you'll want both. Internally you should create and use your own unique IDs to maintain a clean relational model with FKs between your tables while keeping a reference to the eBay unique ID in an appropriate table. If you need to reference something by the eBay ID you will have it available, but should the eBay unique ID rules change it does not affect the consistency of your existing table relations and architecture.