I've had people buy stuff off me to flip a handful of times, doesn't bother me, if someone is getting their price then who cares what they do with it. If upset someone is making money on it, then sell it for more(?). Never really got that, unless the buyer gave a sob story, knocked you down in price using that story, you gave in to them, then saw it being flipped...then maybe.
I was kinda bummed with that. My race car, that I owned for 25 years was sold to a flipper. Somebody called and gave me a story that he was trying to get back into racing he'd been out a while. I wanted to sell it to someone that would appreciate the work I put into the car over all those years. Well he showed up with 2 guys that were "racers" that could help him look over the car. Turns out, 2 weeks later the car showed up on the internet at the track with one of the "racers" driving it with FOR SALE on it in shoe polish. I just keep telling myself I got paid, not much I can do.