Quote Originally Posted by Aodhán
I know that on auctionsites it's more 'what one is willing to pay' that determines the sellingprice ...Thanks!

Can you name anything where the price is MORE than one is willing to pay? I hate to break it to you, but that IS that market value, that is ALWAYS what determines the price. Unless you enjoy having a lens for sale and not actually selling it.
Have a look at KEH's web-site, scan eBay (completed listings), look at places local to you - its the only thing that will give you a picture. Make sure you judge the shape of the itmes against yours, as that can affect the price greatly.