Buyer wants to cancel a sale after winning item

[summerk08]

A buyer has won an item that I was selling and has asked me to cancel the sale as her child bid on the item by mistake, I asked them to cancel the sale from their account but they said they have tried and couldn't as it said they were too late (abruptly) I'm all for working situations out like this but I don't want to be left short! how do I cancel the sale without getting a 'bad mark' (as I have looked in to this) on my account and giving them the option to leave me negative feedback and paying fees? Would it be best to open an unpaid item case to cover myself?
Many Thanks

[siennagoldmgf · Accepted solution]

Open an unpaid case against them and close after 4 days if they didn't pay to get your final value fees back, and so they get a strike on their account.

Using the excuse a child bid seems to be the standard buyers use when they don't want to go though with their purchase.