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After my AddFixedPriceItem (or any other) job completes, do I need to create a new job if I want to submit more listings? Or can I simply upload a new file and reuse the job?
No you cannot reuse jobs. A job goes through the following states (normal error free scenario)
1. Created - via createdUploadJob
2. Scheduled - via startUploadJob (Your job will just sit in the created state unless you schedule it via this call) The job has been internally scheduled for processing by the Bulk Data Exchange service.
3. InProcess - Processing on the data file or the report has begun
4. Completed - Processing on the data file or the report has finished. You can download the response file or the report by making a call to downloadFile
You will need to create a new one each time you want to upload a new payload file or download some report.