Figured it out.
The shipping policy was the issue. I was getting an error trying to create shipping policies, which meant my listings had no shipping policies. On my install, I had to issue the following MySQL command. This line (162) had "Standard shipping from China/Hong Kong/Taiwan to worldwide" for its title. The /H was being mistaken for a regular-expression modifier on line 1733 of app/code/Ess/M2ePro/Block/Adminhtml/Ebay/Template/Shipping/Edit/Form/Data.php. Changing the slashes to commas fixed the issue, and I was able to list on eBay with no issue.
UPDATE `m2epro_ebay_dictionary_shipping` SET `title` = 'Standard Shipping from China, Hong Kong, Taiwan to worldwide' WHERE `m2epro_ebay_dictionary_shipping`.`id` = 162;