The most reputable line of transport in Vietnam is Sinh Tourist. Since many tourists travel between Ho Chi Minh City and the capital Hanoi, there are hundreds of scammers (individuals, corner shops) who call themselves Sinh Tourist representatives.
They charge three times as much for a ticket and are really hard to avoid, especially the ones that sell tickets in businesses such as a convenience store-- which you would think was a legitimate supplier.
How to avoid: before traveling to Vietnam, make sure you find out how to recognize, contact and visit the real Sinh Tourist kiosks and reps.