Lenses (especially good ones) tend to retain much of their value. I am not saying that you won't get one for the price you are willing to pay, just that unlike bodies, lenses do seem not to depreciate anywhere as near as much, nor do we upgrade/change our lenses as like we do bodies - both these points helps keep resale value. £250 for a mint version of a lens that costs £330 (your stated prices) seems pretty reasonable to me.
Good luck with the search anyway
