But you cant ignore the fact that some of these properties may have been rising in value at unrealistically annual rates due to the property boom of the last decade so a 10-15% drop may just be bringing the value back to what it should have been if there had only been normal gains the past decade.
If the price being offered is 40-50% less than what could have been achieved a couple of years back, then I agree, you should not sell.
