Ok, played around with this a bit. If you initially submit the mini with the eBay id field blank it adds the model fine, and you can then add the auction ID to the created CMON image and it works as usual. However, if you submit the eBay ID at the same time as you submit the initial image and title and so on, it always puts in the broken eBay item number of 2147483647 regardless of what number you've put in that form field.