This would certainly take the hassle out of selling a fleet and removing the misery of getting scrap prices at the same time.
I would think that the offer price would have something to do with the appeal of the plane as well and these attributes would adjust the offer somewhere between fair value and scrap. For example(s):
1. Age of the plane (newer plane is closer to fair, older closer to scrap).
2. Economic viability of the plane type based upon year (A brand new DC-6 in 2010 might as well be scrap). Could be measured by number of years since production line closed.
3. General popularity of the type (could be measured based upon production line queue, proportion of planes under 10 years in service vs for sale, new production price indicator, etc.)
4. Number of planes being sold at one time (the more you sell, the less per each you get, as it's that many more than have to be "placed" by AI brokers). Note to do this one you'd want to do a look back over a few years to prevent players from selling 5 per game day/week/month to get the higher price.
As the offered value approaches scrap, there could be a limit where the broker would decline to make an offer or specific attributes where the broker would decline to make an offer.
Then there's the consideration of plane flipping - brokers doing cheap sales could end up buying it back for more. Perhaps a minimum holding period would be helpful here, though, airlines with brand new planes coming off the line that they can't afford to pay for could be a good use for this...