This happens with books too, though in these cases it can be more difficult to spot beforehand (e.g. 20-30% off a cover price isn't too unusual). However, if the seller has _every_ book listed for the same price, something is very obviously wrong.

From those I've seen, it looks as though old and/or inactive accounts have been compromised. These have a decent history, possibly even a positive feedback rating, but no activity in (e.g.) the past 12 months.