Since retirement, and even prior to retirement, I have been buying and selling my way to audio heaven. The fact that I am talking to you now, is proof that I haven't made it there yet.
That being said, there are a few things I keep in mind when buying and selling gear.
Given a choice, always buy gear that is desirable to others and has a proven performance history.
Know the value of what you have and price it accordingly. I try to be around the medium price because getting even moderately greedy only means it won't sell quickly, or at all.
Clean the gear meticulously, take good pictures from every angle and call attention to any imperfections.
Don't modify gear that you don't plan to keep forever. I think modifications of any kind scare people enough to look elsewhere. Point in case. I was looking for an Aurender N100 streamer and it took quite awhile to find one. Despite their scarcity at the time, there was one unit that remained available for months. I can only assume that this was due to the owner putting in aftermarket fuses.
That's my three cents.