You sold it the buyer should pay. No haggling they should have done that before buying.
Wait 4 days and then go to the item in your sold section MOre actions or ebay help at the top of the page and My buyer has not paid.
You get your fees back the non payer gets a NOn payment strike on their account
See the ebay policy
https://www.ebay.co.uk/help/policies/payment-policies/unpaid-item-policy?id=4271
Once you are sorted out then put them on your blocked bidders list so they can not buy from you again
You can go back to buyer requirements and Stop blocked bidders from contacting me.
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We are just ordinary members trying to help.