Not that I have ever bought any, but if they are £10 new why I would I buy secondhand ones from you for the same money ???
You can stick them up for sale for whatever you want, but what you think they are worth and what a buyer thinks they are worth are 2 completely differents things. Whilst junk said he sold a complete kit with a front and back for £50 shows that what I have said seems right.
I don't want to piddle on your bonfire but if I was buying these, the most I personally would pay would be £20.
On eBay at the moment is one insert with a start price for 99p with £1.50 postage. This is something I would buy but at a max of £5.00.
This is only my opinion.
Phil