I recognize the OP is asking about feelings around a machine you are letting go of. I haven't let go of a project machine yet, but I have noticed some interesting feeling patterns around buying and restoring machines.
I've heard wise men say, "You can only Want something you don't have, that is the nature of Wanting. Once you have it, you no longer Want it."
But I think Pride comes from "Accomplishing a challenging set of tasks." And I think pride is best rush of all, because it REPEATS on its own.
I can wake up and randomly remind myself, "Yeah, I did that." And feel great all over again, as if I just did it last night. I might wake up and randomly think, "Yeah, I now HAVE that", but the GOOD FEELING isn't going to be as strong as what I get from a Pride thought -- no question. That's just how it is for me.
So I would say, I notice more of a "post-birth depression" after a purchase than I do after a project.
I hunted for machine X for 6 months, and I REALLY enjoyed the hunt, and the searching, and finally finding the "right seller" (which is the actual "accomplishment"). When it's the night before pickup/delivery... it is certainly like being 4 and Santa Claus is coming tomorrow. I find that to be quite a rush. I enjoy it every time.
Bringing a machine home is nice, playing it is nice, but there's a drop off in happiness for me from Hunt to Playing -- because the HUNT is over. The Hunt/Finding the perfect Seller -- was the accomplishment. I notice this every time, but I expect it to happen, so it doesn't surprise me too much. I don't confuse that drop off, with "OH Sh!T, did I get the wrong machine???" I know my love for the machine may take weeks or months to develop. I give that time.
Now, as for projects. I STILL get a good feeling months later when I look at a machine I put 40+ hours into such as my GL, and I feel a lot pride from that, but more importantly, I feel it often -- without trying.
Hunting = fun
Owning = flat or a letdown cause the hunt is over
Projects = ongoing reverberating/echoing pride (doesn't ever stop feeling good)
I'm new to the hobby, so I can't speak to selling a bought machine versus a project machine and how difficult or upsetting that is, and which might be easier. My hunch is I'd have a harder time selling a project machine. Perhaps, it's like letting go of a friend you spent more time helping out than one you never had to invest time helping out. There's an investment there. But that's only a hunch.