Conversely, I've sold loads on eBay, and I've only been "stung" once, where someone said they didn't receive something I sent. The buyer had relatively low feedback, and I ended up losing about £15. I definitely would be worse off, to the tune of at least £100, if I had opted for signed for every time instead of risking it and going with standard 1st/2nd.
In your situation, I would list items as free postage, then if you get a particularly dodgy looking buyer, or one you feel dodgy about for whatever reason, then you can choose to spend the money to cover yourself for that specific instance. If I were you, I wouldn't even begin to think about blanket sending at "signed for" for such low value items. It really isn't worth it in my experienced opinion.
:edit: Only do the above with the knowledge that if you don't send at least "signed for" then eBay/PayPal will side with the buyer in any dispute. IMO/E, in your situation you'll be much better off risking it with standard despite this.