MarchingToTheBand
So if I wanted £25 I would list it at £30.
I'd definitely advise this. I've just sold a pair of shoes for £30 expecting they'd go for £25.
I'd had, and happily accepted an offer for £25, but the person offering didn't get back to me (why do people bother to do this?) so the extra £5 was a bonus, but often people like to feel they've bartered you down a bit.
I may have underpriced the first thing I sold which I'd bought for £250. It was still available for £250 new when I sold mine and I'd only worn it for about 6 hours at a wedding (I changed for the evening).
That went immediately for the asking price, £75, and I hope the buyer felt they had a bargain 
I suppose my thinking was to ask for about 1/3 of the original price, but with more experience, I might have asked for £100 initially.
The shoes I mentioned were priced for roughly 1/5 of their original price, but are a few years old and there's a bit of wear and tear to the heels.