Since my retirement I have renewed my interest in audio. I have bought and sold many audio components on AG, and bought on Ebay. This activity fostered development of 4 systems: speakers, DAC, cables for my primary system in a dedicated room; all components for my secondary system used outside on deck; most components for my tertiary system which supports video and audio in living room; and those for my headphone system. I have swapped out equipment in the development of each system, however once I reach 'evolutionary climax' with a system, which has been a 3-4 year process for any system, I tend to keep it with minor changes for several years. This is now true for all but the headphone system which I might change out regarding the amp/DAC configuration. Prior to retirement I owned the same system for ~25 years. As with the aforementioned ones, it also took about 3 years to develop.
I won't try to speak for others, however in my case, I buy and sell with the goal of developing a audio system for a specific environment, possessing a sonic synergy for that environment, and once developed to satisfaction enjoy that system for a long time.