Knowing exactly what you have could make a difference on prices. For instance, most Tyco cars will sell for $1-$2 each, however the specialized ones that have some sort of automated loading/unloading can sell for $5-$10. I've seen rare ones easily sell for over $50.
You mentioned the unloading hopper, which is a specialty of mine... If it is the common Virginian then you might get $10 for the whole package with the unloading track. If you have a Brach's with the top and the paint is in good condition, you have a $50+ car. There are several different paint schemes, and variations of some. For instance, there are three regular variations of the blue B&M hopper. So details matter!
You'll find similar pricing differences with the locos, but taking the time to clean the motors and make sure they are all in good running condition can increase the price significantly. Not many people will put down good money for a loco that may or may not even run.
The buildings probably aren't worth much. Some people collect the original models still sealed in the box, but I see the assembled ones all the time at train shows going for a couple bucks. They are really only interesting to someone who is trying to build a layout with equipment from the 70's.