If the snake must be an import, be very careful when it comes to selection. A docile specimen should move forward and flick it's tongue in a regular fashion. A truly defensive Boa will make it's intentions very well known. Normal actions are pulling into a strike position and hissing. A specimen will most likely calm down with interaction, but a calm specimen is a much more enjoyable pet.
Hunger folds are another sign of malnutrition.
Although it is generally not a problem with Red Tails, check to see if the specimen is a known feeder. Ask to be allowed to watch the next time it is fed, maybe even offering to purchase the food. However, I would wait at least 1-2 days before moving a just fed Boa. Mites are no reason to reject a snake, but are definitely something to look out for. I have seen neonates ranging from 12 to 24 inches.