Talking to myself here, but I'll not be deterred.
So, I only have access to sections when the item is listed for the public. But then to list publicly, I have to open it for purchase before I set the markup. To set the markup, I have to use the CSV, as I can find no option on the model page for that. The entire point of the CSV to to allow efficient alterations (e.g., markup to all models or something). But if I wait until I've activated for public sale all 240 of my models, each one done one at a time by hand in a slow process, then use the CSV, people could buy models before I have any chance to set a markup.
This just all seems crazy. I really hope that I am missing something easy here.
Seems I will have to first, one model at a time, make it a private listing (x240), then download the CSV, markup, reupload. Then, one model at a time, set it to public, one model at a time move to a section. So 240x3 over 700 iterations!