FWIW..here is my approach to buying used stuff at a "good/fair" deal, kelly blue book aside...
Deducted from retail price:
NIB -10%
Used but very good condition (low use) -30%
Used good but cosmetic wear and/or moderate use -50%
Used serviceable but <50% life remaining and/or needs repair/PM -70%
Albeit, every used purchase price is very subjective and relative to supply/demand.
Also, I'm a fair deal guy. Not a "haggle a guy down so low that he wouldn't stop to help me with a flat tire in the future" type guy.
If retail on something is 100 bux, and its been used lightly I'm happy to pay 70 bux for it. If a person is asking 70 I'll give it to him. I will not offer him 45 and haggle to 65--life and time is too short for me.
On the flip side, treat people the way you'd like to be treated. I offer thing for sale a fair price and often ignore low ballers. And if you are dealing with a firm, do you not expect a company to make a proffit and stay in business? Thus why so many retailers have had to go internet sales only.
Sorry for the rant..... But, in short, do you need a part or do you need bragging rights as to how low a price you got on the part?