I'm a web programmer by trade, so the evaluation I'm giving is based on how I would actually go about implementing something like this. And a barcode-reading app would be considerably more difficult. Then of course you have the issue with the used game's market itself. These days, its very common for a lot of used games, including disc-based games, to not come with their original cases. If you don't have the original case, you can't scan the barcode. The kind of system you're describing would be much better for adding items to a database, rather than actually searching.
And when it comes to searching, I think implementing voice commands would be even faster/better. And since some mobile API's are actually having voice commands integrated into their standard libraries, it's also much more feasible from a development standpoint.
Adding the option for connecting to an on-line database is fine for those individuals who do have unlimited data plans. But offering the option of backing up a copy of a database to your phone's memory is good for those who don't. An app that offers both options would be the best approach, allowing a greater number of choices to the consumer.