Cardboard boxes are fine. Most brand new bikes are delivered in one!
Before you start, get the couriers dimensions and check your box. I had to go all blue Peter on mine.
Also, work out your postage costs before listing it.
I sold a road bike for £800 on ebay. I sent it via parcel force. I insured it. I think it was something like £5 per £100. I had quoted £50 postage and packaging. Took me 2 hours to package it and take to the post office. £50 only got £600 insurance. Parcel force were excellent.
It was a hassle and a bit if stress posting it, hoping it arrives OK etc but I sold it straight away and got far more for it than if it had been collection only.
Just make sure you pass the costs to the buyer and make sure they are aware before they buy. Try and list on ebay when you get an offer otherwise they will take a huge chunk.
Don’t forget singletrack has a free classifieds! 😜👍