Because that's what nits do... When they see that a guy has more than one high quality cue - they think he is rich. Then they think that he can afford to let this one go cheap. I have nice cues but no money. It's because I saved for years for them. I have also bought a bunch of mid range cues and trade 2, sometimes 3 for something of great value. Truthfully I think that a lot of nits are just ignorant. They couldn't tell a Rauenzhan from ravioli...
If I'm a seller and an offer is too low, I just say "no, thanks."
If I'm a buyer and the price is too high, I just say "no, thanks."
This method covers all situations and I never find myself upset. Try it, it's good for your blood pressure.
If saying "no thanks" is too troublesome, then a really strong disclaimer on your for sale posts would probably cut out most of the offers that annoy you. Something like "This is the price. Only contact me if you want to pay the listed price for the posted cue. Please do not contact me with a lower offer, a trade proposal, or a story about your hard luck."
Cory