Is this the Nintendo? Your buyer didn't pay the £5.88 postage price, did they?
If your buyer collected they should have been issued with a 6 digit code from eBay which they should have given you when they collected - this proves that they have their item and they then can't claim their money back by saying that they haven't received it.
I'd message them and ask them to check their emails and eBay messages to see if eBay have sent them one.