Do you know there is a thing called Proof of Posting, Royal Mail call it a P6159 certificate. This is for standard parcels that are non-urgent.
Why should I bother and what are the benefits?
Free Insurance up to £32.00 or market value
It doesn't cost you anything extra to send a parcel using Proof of Postage
But the main benefit is you can reduce the postage cost on your auctions and still be protected with the insurance. This would make your auction a little more attractive than some I've seen.
The only disadvantage is delay in delivery 1-3 days.
Whilst I wouldn't want to hang by my neck for two or three days what's the rush; if your experience is anything like mine Next Day Delivery has arrived two days late.
I've added the link to the Royal Mail web site that gives you all the information you require, www.royalmail.com/portal/rm/jump2?catid=400028&mediaid=400031
NB: Just copy and paste the above link into another web explorer page, links outside ebay are not allowed, hence this note.
If the Item you are sending is valued at more than the £32.00 then of course change how you send it; they have other services which cover higher value items.