Just apologise, explain why and refund the buyer if concerned.
Otherwise providing you ship it royal mail special delivery and add the tracking number on the sold item, eBay pulls through delivery confirmation and you will prob get funds released in 3-7 days. RMSD seems to pull through the eBay system much quicker than other delivery options and some couriers never get recognised. Even quicker is if you book the RMSD via eBay print label service. Usually they release the funds within a couple of days of delivery receipt.
You may not like it, but that's the way it is.
They do this on items for a number of causes including (not an exhaustive list): a) if you don't have PayPal balance on account which part or fully covers the goods value in addition to the item that's just sold, b) if the buyer has a new payment card, c) if the buyer has raised disputes or instigated chargebacks previously, d) if you have raised disputes as a buyer or seller, e) if you withdraw money from buyer to your bank immediately every time you get paid and f) if you or the seller just don't meet their risk profile.
It's all automated inc the link with your eBay account so you can't win on this.