I do it all the time. I also question why sellers price things at retail when they are used, then complain when you offer them less. It's a personal FS/FT forum, not newegg. If you plan to stand behind the product and have the same resources for RMA like newegg does, then fine charge retail. Otherwise, expect to be offered less especially for USED items.
Also, what YOU think is a reasonable price may not be the same thought the buyer has. For example, I see iPod touches sell here all the time ranging in price from $75 to $150 shipped for your average 8GB model depending on generation and condition. Personally, I have purchased three at $70 each, so why would I pay $150 just because another seller thinks his price is "reasonable or fair" when I've gotten them MUCH less from other people?
The primary issue seems to be that often sellers are not doing their research before pricing their items. They pop on newegg and see an item, they price similarly and expect to get that amount. If it's me, I'd rather pay a 10-20% premium and get an item NEW from newegg knowing if there's an issue i'll get an instant RMA and refund/replacement rather than dealing with a possible seller giving me the "it worked when I sent it" line.