I'll only sell off a piece if the re-sale value if good. If not, i'll just keep it and try my best to appreciate it.
However, if a piece is doing well and it's time to sell it for a profit, then of course, i won't hesitate. There aren't many keepers nowadays anyway and i've noticed( and even sold) watches that are simple ETA based models but fetching a high resale due to the design etc. Watches with ETA movements don't rank highly in my eyes.
About keeping pieces, i keep pieces with manufactured movements.