It can be a tough call, especially when you involve a passion, I would always find myself breaking my machines into 3 catagories, Collection, Shop, and Training.
Collection was usually not about keeping to a certain value even for the potential sale of a restoration, at the end of the day, it was about projects for me, and yhe time spent was enjoyed, and reward in itself, as of course was ghe finished product, kept in most cases for myself.
Sale was anything that came in, was cleaned, repaired, and prepared for sale, so it always had to be worth it, and some repairs just arn't.
Training was interesting, this usually involved models thst were not rare, but offered th opportunity for me to gain knowledge or experience in repairing them, this usually meant running at an overall loss on the machine, but furthering my abilities, or even just satisfying curiosity.
