I looked up NADA base low retail, which is usually the most a dealer will put in a used bike, in REAL money. He may tell you he's putting more, but he is making it up on fianancing, or extended warranty or "Dealer fees". So on your bike it is $12125 tonight using a central florida zip code.
By the time he deducted for your handlebars, seat and windshield, not counting the aftermarket exhaust and the fact someone has "messed" with the computer,, the fact he offered you only a grand less I understand. Not saying I agree, just understand.
We spend big money for upgrades and it used to be OK with the average used buyers, but not anymore. They want a used bike nobody has messed with. I traded my bike just like yours on my 15 RGCVO, they looked at it, gave me a price for original equipment. I went home and spent two days taking stuff off and putting it back as stock as I could. Sold almost all the stuff I took off on here and ebay/craigslist and did OK in the end. Even ended up selling the TTS to the new buyer. So maybe you could try that.