I personally try to offer decent prices on items I know I can sell fast.
Something that is going to take me days-weeks to sell though I can't be bothered to buy from someone unless the price is really low.
An example of this would be something like a 512 head text. I'm willing to pay 17k-18.5k depending on the current market price. I know they sell fast so it won't be a problem. Buying something like an unwanted 3D item or force, I won't dare pay for it if the price isn't significantly lower than the market price.
However, I'm never rude about prices (at least I don't see me ever being rude about offering, etc). I don't know why some people get so mean about prices though when they don't get what they want to be offered or someone is being too high on their price (above market, etc).