I have correct code (custom PHP) working in the sandbox environment for the inventory API and am able to create a listing and an offer and publish the offer, the item shows up in the sandbox site and I am able to purchase it using another test user. However, the same code does NOT work in production, particularly the getInventoryItems call, which doesn't appear to be working correctly using the production API call in the API Explorer on the eBay developer site either. I also think it's insane for eBay to expect/want us to develop selling applications for these APIs and charge 75 an hour for premium support. What is that?! Anyway, I really hope eBay will take a hard look at their Selling APIs, because I have clients that want me to build them working applications now, not next year.