I don't see why a buyback would be an issue whether the car was stock or not. I doubt the dealership is barely even going to look at the car if they do a buyback. 99% of dealerships wouldn't even notice.
Because it's missing parts that substantially change both the car physically and operationally from the time it was sold. But who knows. Even if I was interested in a buyback, I'd be removing a lot of stuff, like the RNS-510, suspension that I added in, etc. I'm sure they'd probably notice that stuff.
I brought mine to a dealership recently to have an alingment (the only time it sees a dealership) and they didn't even know I had done anything to it until I told the tech. He was like "I thought it seemed quicker and smelled a little different". Then once I told him what was done they all thought it was cool. Then the salesman tried to get me to trade it in lol
Of the 3 San Antonio area dealers I've been to in the last 4 months, most of them think it's actually pretty cool what I've done to it. The general consensus I get is that those who modified their cars with a tune/emissions delete aren't even thinking about trade-in's/buybacks period, but this is just what I'm hearing at the dealer.
As for the dealer noticing changes - when I had 23O6 flashed before heading north to get the DPF delete/tunes, I had a crappy Neuspeed Stage 1. Dealer had no clue (and I didn't tell them, since the fine print for 23O6 was that the new ECU firmware couldn't/wouldn't be applied with any ECU/hardware changes), and was able to do it anyways.