We have a few clients using ION to manage in-store sales and TCGP Pro to handle online and it sounds like its a suitable workflow for the stores who opt to do so. We're pretty hopeful though for that API access; a big part of their conversations have been on opening the market up and not allowing the bottlenecks of the past to persist.
In terms of products; yes, you have full control over your inventory, which means you can develop any number of unique products. We had been offering pricing on video games through VGPC, but there have been some recent changes and only two approved POS systems to provide their data directly so we have suspended that. Beyond that, however, our DB has a great deal of titles available, we use a series of tools to produce information on new titles, and we're developing our own price feed and speaking with other, similar, service providers to reinstate a video game price feed. That said, the system handles CSV imports like a champ; its the preferred method for bulk updates and management, so if you have a source, even including VGPC, you can use that to manage your own account.