It's not the single-item undercuts that bother me, it's when people put up 99 stacks of HQ items at half the market average. For many items you can still salvage the situation by, as you say, buying and re-listing. For other items that's not economically viable for a lot of people and before someone with a bigger wallet comes around to fix things, the damage has been done and other listings have made the undercut the new baseline.