This paragraph of yours leads me to believe that perhaps you aren't familiar with the current status of the used high-end lens market. .
It is quite easy to look up the prices that used lenses actually sell for, either here or on eBay or on other forums. . If a $2,000 (new) Canon lens commonly sells for $1700 or $1800 used, then that is the going rate. . Remember, the market is set by supply and demand - no other factors are of any significance. . So if a big bunch of people are willing to pay $1800 for a lens that was $2000 when new, then yes, it would feel like someone was trying to take advantage of me if they offered me $1,000.
Market norms already exist for used items - we don't need try to come up with "what should the price be?" because the price of the used item has already been established by all of the hundreds of other recent sales of that exact same item in the same condition, in the same country. . Because these market norms are so well-established, we all know what the going prices are for any given lens. . Therefore, it seems like someone is trying to fool us, or catch us off guard - when they suggest a price which is so far below the established norm.
When somebody gives a super lowball offer, I feel as if they are saying to themselves, "hey, maybe this seller is dumb enough to let this lens go for way, way less than it is worth." . Or, "maybe this seller doesn't realize what these lenses are going for these days - so perhaps I can get a real bargain off of him." . So the fact that someone is acting as if maybe I am dumb or ignorant is actually rather offensive.
"Your" and "you're" are different words with completely different meanings - please use the correct one.
"They're", "their", and "there" are different words with completely different meanings - please use the correct one.
"Fare" and "fair" are different words with completely different meanings - please use the correct one. The proper expression is "moot point", NOT "mute point".