I accept offers on my stamps, and most buyers that make offers play the game and make sensible and reasonable offers. Today I received an offer on a stamp with a missing colour that has a catalogue value of £80. I have it listed for $55, or about £27. The offer I received was $30, or about £15. This buyer wants a scarce missing colour stamp at under 20% of catalogue.
I very politely counteroffered at $48, stating that was going to be my best, then he came back at $38, which I declined.
This is a technique used by some buyers, where they try to wear you down, hoping you'll meet them "half-way" by which time you've lost 50-60% of your sale.
I'm not in the business of giving stamps away!