I’m sure that the ‘estimated delivery time’ on a listing to a degree reflects past performance, the the help info backs that up.
As mentioned above you can change your settings for your ‘despatch’ time
www.ebay.co.uk/help/selling/posting-items/estimated-delivery-dates-sellers?id=4086
I aim to send on the same day or next day from payment, but I have not specified that and left it as 3 days.
This covers me if I fail to do so. I try to avoid my auctions ending on an unsuitable date, but I often accept offers which means I could sell at any time - recently I sold an item within hours which included offer, accept and payment
I’m often away for long weekends during the summer etc, which at its worst could be Thursday afternoon to Monday night
But Ive also fallen foul to Vinted ‘offers’ thinking at first that an offer indicated I was about to make a sale, so Ive rushed home to pick up an item making sure that I’m able to post it if paid while I’m away …. And then heard nothing again
(Even with one item I was asked on 18th October if I could post by Friday as it was needed for Sunday the 30th ….. it’s now 3 weeks later)
I have not yet set any holiday periods, but I have messaged buyers if I’m going to have a delay such as a Saturday sale that I won’t get out until after the weekend - On every occasion the buyer has been happy with no urgency.
On one occasion I sold an item which the buyer was happy with the 2nd class postage but did highlight that they needed to allow for adjustments ready for a function - just reassuring that I can get it on the way quickly.
At only a little extra it was worth it to ‘upgrade’ to 1st class, and as a result protected us from delay and for me a delighted message back with appreciative feedback
If you set a realistic despatch time and add a footnote in your description then you’re being fair to buyers. Consider multiple postage options, eg you post on Fridays with 2nd class as your default, 1st as an extra option
Then perhaps allow for someone to message you and ask nicely for something quicker - it may turn out to be convenient and you manage it or not