Know a few people who buy cars and bikes which have an insurance marker against them. All depends on what catagory it is really. Some are purely cosmetic damage while others are actuall write offs and need to be tested properly before being allowed back on the road.
If you know the seller and trust the details of the damage then there is nothing wrong with buying one as far as saftey is concerned and you can get some really good deals. However, like said above, insurance will be affected by cars/bikes with markers and some wont even insure it.
As for resale, you will always be able to sell it on so no worries there.
Another thing is i know plenty of cars which suffered worse damage than some insurance claims and were never recorded, so all because a car doesnt have a maker against it doesnt mean it has never been wrote off/damaged!
Mark