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It happens to us all the time.
We've posted furniture or other items on Craigslist, and people always email us saying how much they want it, and say "I'll give you ($10 or $25) less for it."
I understand that some people sell things at inflated prices, but we don't. We have sold things for 1/10th of the price we've paid for them and even if we put something up for $10 someone will say "I'll give you $5.00."
If we put "Or Best Offer" I could see trying to bargain! - Ebay allows you to check a "or best offer" box, and since I have sold comics from my collection, sometimes I do check that box.
FREE, FREE, SET THEM FREE: The other thing we've learned is that if you post something
"Free" on Craiglist" within 4 hours we'll get 10 to 20 emails begging for it. We did that with carpet rods for stairway carpeting when we had our stairs redone. In 2 hours we had over 10 emails asking for the free hardware!! I guarantee if we posted it for $5 there would have been no one, or people asking to give us $1.00 for it.
THE SUGGESTION: The next time you answer an ad where someone is selling something, ask them in your email to them if they will accept a "Best Offer" instead of undercutting the price they're asking before even seeing what they're selling. It's the right thing to do.