when the court papers arrive, there is a space to put details in if you dispute the claim. Simply fill it in with everything you've put in your post above. Attach copies of the listing, all the emails, the pdf court notice etc etc. This will then be sent back to her and she can rant and rave and resubmit with reasons why you SHOULD pay, but all this can take months.
this will be costing her over £30 to issue the claim, she is obviously a nutter. NO-ONE with even half a brain spends money UP FRONT to dispute £15. You would only have to pay the extra £30 if you admit fault.
When you do not pay, her options then are to let it go, or to actually take you to a court date, which has to be at the nearest court to you, and thus hopefully inconvenient to her. I am pretty sure that the costs of this court visit have to be paid by her, and added on to any winnings that she may get from you.
I have been to one such sitting of a judge at a small claims thingy. I accompanied company's in house solicitor. The judges see everything under the sun and quickly work out when one person is being unreasonable and bullying. Honeslty I don't think you have anything to worry about. She will end up with all the costs etc.
However, if you pay even one penny, this throws the whole thing out.If it were me I'd now paypal her, or send a cheque recorded delivery, for the £5.30. That way to get the rest she has to start again with the whole court costs and papers and everything, and you have proved indisputably what a reasonable person you are.
I have 800 ebay transactions and only ever had one or two 'difficult' ones and one that I reported to ebay for FB extortion and they came down firmly on my side and gave the seller a warning. So its not totally full of nutters!