Here are a couple of more strategies of swindling customers that you should be on the lookout for. Now, I really don't know how prevalent this kind of practice might be- they are not totally rare. But that doesn't mean that there aren't a lot of really honest people in the garage door service industry who want to give you a good value for your money. Distinguishing these people from the crooks requires that you maintain your wits about you and closely observe.
The "liability" Trick: this is the other half of the "safety" trick. It's not put to use to sell you something necessarily, but to help persuade you to go for a more expensive option when you wouldn't normally. If you just need a new spring, they might try to get you to go for a whole new garage door. But don't be too quick to judge if the word "liability" is used- older garage doors can be legitimately dangerous, and there are lots of genuine issues of liability for garage door service companies.
The "disaster-is-coming" Trick: The garage door service guy takes a careful look at your door, pacing seriously, and then brings your attention to 'cracks' in the middle of the garage door. These are common and generally appear in the middle of the door where the panels have to bear a lot of weight when the door is suspended up. You may have never noticed these cracks before, and this realization will make you feel panicky and open to suggestion, especially suggestion to buy a new door. This is not always necessary, so ask a lot of questions and judge for yourself.