Craig,
you are right that there are more expensive options around. If your design course includes marketing, then you will need to show that you have thought about who buys the stuff you have designed, what they bought before your product became available, and why they will buy your design instead. Is it cheaper, is it better, is it trendier, etc.
What problem are you trying to solve with your design? I use an old sports bag, that fits in the buggy seat as I pull it to the beach. Stuff doesn't fall out, and I don't have a problem without axle stands. Your design will have to offer me something I need, as well as being cheaper than than the alternative.
Cheers, and keep designing stuff. It's more fun than just buying it.
Ben