While I agree the quotes are sometimes crazy low, they do say in their descriptions that their buyout prices are usually 50-70% of the price they expect to sell for. I'm guessing they are calculating their expected sale price based on 1 or even 2 markdowns, too. If I have a very high value item, I'll always try to sell it myself first. Otherwise sites like FP are a great backup.
The thing about FP that REALLY gets under my skin is the over-retail pricing on items you can literally buy online brand new from the retailer if you are patient (YSL uptown pouch, LV multi pochette, etc.). Doesn't apply to Chanel but it still really bothers me.