What Does "Reasonable Offer" mean?

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My mom used to sell trinkets at a flea market. When the people made half price offers, she RAISED the asking prices.
How much for the dinner plates?
Those are $15.
Okay, I'll give you $8.
Sorry, they are $18.
What? I thought that you said $15 ?
Okay, make it $20.
She was good at detecting cheap people so she decided to mess with them since a sale to them was a losing effort anyway.