Fix Marketplace Tests

[dr-Jess]

Current marketplace tests have some critical issues that need to be addressed:

• Running SQL queries(!)/trying to delete items in setup--should rely on the Django test runner deleting items, and assume ids are not necessarily sequential and starting from 1
• Test cases expect ids to run 0, 1, etc: should dynamically capture the ids as needed, and reuse the captured id when asserting (see sublet tests for how this is handled). The test runner runs tests in parallel, so no matter what this is a dangerous assumption
• Mock data files are jumbled/there are redundant files to be cleaned up.