From this point onwards do everything in writing.  That way there can be no contest as to who said what and when.

Firstly send the buyer an email advising them that the item had been posted and requesting payment within 7 days. Keep it polite and business like.  At the same time obtain hard copies of everything; the eBay listing, all correspondence and notices received from eBay in connection with this sale and any e-mails received from the buyer. Also make a record of any and conversations you have had with the buyer and notate them as not contemporaneous – that is, acknowledge they were not compiled at the time the conversation took place, but instead compiled from memory some time later.

If payment is not received within one week send them A “Second Notice” requesting payment within 7 days.

If payment is not received send a “Final Notice” demanding payment within 7 days and with a warning that, if payment is not received with 7 days you will commence a debt recovery action.

If payment is not received go the Registry of your local small claims court, or access them on line.  They will then advise you of the procedure you need to follow to commence the debt recovery action.

Yes the above requires you to put yourself out and do something positive to protect your interests.  The benefit, as long as you can prove you sent it, they (the buyer) will not only be ordered to pay for it, but also compensate you for the court costs you incurred in bringing and pursuing the application.