Pretty much what Shannar said.
I think complete armies sold as one lot only fetch a high price if it's all assembled and painted well. If you have an army that matches that description and some rare models in there, then you can expect to get more then the RRP. If not, then I think the price drops very fast. If you break it up into separate lots then it will depend on the model. Stuff still in a box that is still in production will get between 50-75% of RRP once shipping is included I think. Models still in box no longer in production will depend a bit on how popular they are but will fetch around the RRP. Rare models (think metal everqueen) will usually fetch a silly high amount. Assembled and / or painted models really depend on how well they are assembled and painted. Pro-painted models can fetch more then RRP. But then they really need to be very well put together and painted. Otherwise, expect 50% as the high point.
A good starting point is having a look around ebay what others are asking. And also ask yourself how much you would be willing to pay for a collection of models.
Please try to remember that, no matter how 'official' the source seems, rumours are basically just a dictionary combined with a random number generator
For Nagarythe: Come to the dark side.
PS: Bring cookies!
Check out my plog
Painting progress, done/in progress/in box: 166/17/95