It might be worth insuring the full amount and requiring a signature for delivery. It adds $15-20 to the shipping price, but with something that expensive, I'd say it's worth it to ensure that you aren't taken advantage of if the person decided to be shady.
Insurance will do nothing. I spoke to both USPS and UK couriers about this, and they said that for something like a collectors item (e.g a clack or rare keyboard) the chances of getting a claim were next to nil.
In most TOS for insurance it will say something along the lines of 'market value must be easy to ascertain', and a single ebay auction doesn't fall under that.
The best thing to do is box the clack multiple times, each one very well sealed, video yourself packing it in the post office and continue videoing without cuts until the tracking number is visible on the receipt.
I bet they have no problems with accepting peoples money for insurance that would be useless from the start. But then again it really is up to people to read that sort of stuff, even though that information can be lost in a sea of other seemingly pointless information.
When the Model M Industrial keyboard I got from the US arrived damaged, I put in a claim with the forwarding company (different to shipping of course) they paid out the $180 USD (total package value had other stuff in their also) value of the package. Although this wasn't easy to do. Firstly the only method of contact was to type in a word into the search for answers page which kept defaulting back to the original page when you hit enter. So I searched them on facebook and made contact with one of their departments which sent me an email address. I contacted that email address and they advised me that I had 7 days from date of receipt to package to lodge and gave me another email address. I contacted that email and provided all the information photos etc and they sent back a generic email asking for almost 1000 photos of the item, the labels, the box, the packaging material and detailed description. This was about day 5 by the way. They then approved my claim and it took about a week to get my money for the item.
Out of curiosity I just checked their website and they have sorted out the contact issue, I may consider using them again if the need arises.
tl;dr theoretically there may be some company which would provide cover for an item like a really expensive key cap. Because seriously who in their right mind would think that an old keyboard would be worth more than $20.