Fred I'll bet you 1k right now that there is no way in hell you'll get 17,156 for a 39 mpx sensor used. There selling for 12k from a dealer so in effect they are throwing you a extra 5k discount towards a upgrade. There is no better deal when you upgrade to a more expensive back than going through there upgrade path. You'll never get a better deal on your back than what the dealer will give you on trade value for a upgrade. Selling and getting out all together than obviously private sale is better.
Walking in with cash on the barrel head you won't pay list true but you only got maybe 12k for your 39mpx in a private sale. Do the math it's the same and much more hassle. Go try selling your back. Good luck with that one. Your competing with dealers that are offering warranties, service and support. On a purchase that maybe worth 20-40 thousands dollars most folks are looking for warranties , service and support. Its why the dealer network is there in the first place. You want to buy on e-bay at maybe a discount than you basically are on your own. People will weigh those options and it will be there choice. But I will say this seeing numbers out there as list than consider it list not what you write a check for. Its a MSRP stamp