Laura, You have my sympathies, I was in the same position, also with a Nintendo DS, although I paid with Paypal, so I'm now persuing them for a full refund. The problem with private sales is that you have no right to good quality - although you do have the right to expect the item as described. If you have the sellers full contact details, you might want to think about making a small claim. Otherwise in future, always insist on paying via Paypal because you will be protected if things do not go acccording to plan.
Comment on It’s second hand – do I have any rights? by Catriona