Wow!! You opened yourself up for a world of hurt by offering to look at this....
I'd say just take the easy route out and give it back
If you want to continue to try and troubleshoot it then I would suggest:
1. Connect the battery directly to the motor and make sure it turns.
2. Start with the gears and axles, stripe them down, make sure nothing is rubbing, lube it all up.
3. Check the board for any obvious broken wires.
From the pictures this thing doesn't exactly look hobby grade so unless you can find a circuit diagram of the board you aren't really going to be able to do much with it.
Not really sure what else to suggest.