Pay a used Price for a Used Lens
Having bought 3 lenses and a second camera in the last 2 months I can tell you that prices are quite low. You shouldn't be paying that much of a percentage for a "used" lens.
People buying used equipment because they either:
- Need/Want something specific
- Are trying to save money
And with the economy most buyers are the second category, and they are really trying to save money. Most sellers get that and know that its currently a buyer's market out there.
That said there are quite a lot of individuals advertising used equipment at 90 to +100% of current value in the stores because they paid more for it 1 or 2 years ago; and they have had that stuff for sale for months. I know of at least one or two locations that keeps selling similar items and they are likely buying used and trying to resell at a profit; fact of life but do your research.
Also I generally suggest you avoid the "for sale best offer" with no price mentioned, often its someone trying to figure out what their stuff is worth, and may not be an immediate seller, you could let another deal slip buy while waiting on someone who is uncertain of their asking price. I once had someone say they didn't know the price, so I offered them $20 and they quickly came up with a more reasonable number and we got on with business.
There are also some scams out there offering really good items for pennies on the dollar (like a D3 for $1000). If someone is offering below 50% for a newish item, likely its a scam or broken; don't fall for it.
As for how much a particular piece of gear is worth, its all subjective. However here are some good resources:
www.photoprice.ca - Current Best Prices for New Gear at reputable dealers, you should be paying about 75% of this number MAX. (If your Canadian, I'm sure there is something similar for US customers).
www.ebay.ca - Remember to account for currency conversion, shipping and taxes etc. Generally take the price of the good that has the lowest bid on it, watch a few similar items to see what they sell for. Buy it now prices should be at the top of your scale, as many of these auctions roll over as the item is not purchased, yet sellers always like to quote them as the "ebay" price because they are high.
Also take a look at kijiji and craiglist to see what items are advertised, and if there is a suitable substitute at a lower price. Or your local forsale site.
Remember to double check the version you are buying, some lenses and cameras have several different versions.
One final note: always check the gear before agreeing on the final price. If you don't know how, bring a friend who does. You want to make sure you get value for your dollar.
Remember you don't have to buy it if you don't want too, but if you say you'll buy it at a particular price over email/phone and on inspection it in a condition more "worn" than you would like you've likely find the seller less accommodating to lower the sale price a few percent as you already agreed on the price.
Good luck.