For houses built around the 1995 median, we're mostly dealing with standard 7-foot residential doors on torsion springs sized for a single-car or double-car opening - common sizes, and we carry those springs, rollers, and lift cables on the truck. Drums and end bearings for that same era are stocked too, since they wear out together with the springs more often than not.
Where we don't stock parts: odd-width doors, discontinued opener control boards, and anything custom-ordered when the house was built. If your opener is an older board that the manufacturer no longer makes, we'll tell you that on-site rather than quote a part we can't get in a reasonable window - at that point replacement is usually the faster and cheaper path anyway. We'd rather lose twenty minutes explaining that than have you wait two weeks on a part that may not even show up.