Before I put any issues up I do some research and find out what others are charging, then I try to undercut those prices. If my issues have damage, I start chipping away at the price. However, there's a limit below which I'm reluctant to go.
You might also want to look at postage prices too. I send all my stuff signed for. It's a bit pricey but it's worth it, and it also means there some element of protection.
When I started eBay many years ago, there were a few things that went missing in the post (yeah, right!) and I had to reimburse the buyer.
Funnily enough, no one has made a claim against me since I started using recorded/signed for. Odd that, isn't it?!