I have not sold much, I am more of a "if it does not spark joy I give it away to someone who might cherish it more"-kind of guy. I think it stems from that I enjoy seeing stuff going to people and places where it gets to shine more - and also because selling things is a hurdle and a hustle for me. Often it takes more energy and time listing and shipping something than what I feel I get back for that effort.
Regarding what I have given away or sold I feel like I need to be mindful of why those items ended up flowing away from me. It could be a need to shrink a collection, an abandoned project, or regretting a purchase after awhile. Even if I have found myself second-guessing my choices one time or another I always end up accepting it all over again since I am at peace with the fact that at that particular time when the miniatures found another home it was the right thing for me to do.
Also, even though I have already collected enough things to never be able to fully beat the backlog I have at least become a little better of not giving in to the marvelous madness that is buying everything I need for every little creative idea or project that pops up in my mind. Yeah sure it is cool that I can imagine a certain army rocking a certain theme - and I know exactly how I would convert it and paint it up - but actually following through to the end is hard when that newest impulse purchase competes with all the already ongoing projects that I have. So just because I can imagine what I would do with certain miniatures that I am keen on collecting does not mean that I should try and get a hold of them. This is stemming from having been there way too many times. So now I find myself having to slim down the number of purchases I make, and also try and hold back the urge to buy new things (and also hold back the urge to yearn for the things I have decided to let go).
I have heard that one should never sell anything that one has already painted since it will only bring regret but I find that to not be entirely true. However, I tend to keep the things I have painted as a sort of vehicle for remembering times, games, and people that I have played with. A form of nostalgia, to be sure. And painted miniatures can always come in handy in some way down the line, I reckon. Whether it be rebasing them to play a new kind of game or using them for (at least to me) unforseen purposes like roleplaying games or similar.