I've only ever done a clean out, now almost 20 years ago (1997).
I had been in the hobby for nearly 10 years at that point, and after doing my time in the service, I decided to go to uni. And as I would be moving to a different city for that, I cut loose a lot of stuff which I thought I'd not need anyway (took up room in my boxes and was worth selling).
One of those things was my beakie Dark Angels army. All painted in enamel blacks, with red markings. Pretty horrendous by my current standards, but hey; it was my first ever miniature army, even though I had at that point, never even played one game of RT/40K.
I sold the entire army (about 40 marines, a dreadnought, 3 Rhinos, 10 scouts, 10 Space Hulk Terminators and assorted HQ etc minis) for the princely sum of Fl 100,- (100 Guilders, roughly €45,30 in today's money).
Mind; I only sold this army, not the bits, remains, hobby supplies, my other games (Hero Quest and Space Crusade), my Esher Necromunda gang and all my books and magazines.
About 2 months into my first year, I discovered to my delight and horror that of the 30 or so first year students in my group, about 10 actually played WHFB, 40K, Necromunda or some other GW game. Delight, because I had finally found a likeminded nerd-herd. Horror, because I realised that for a measly Fl 100,-, I had sold an army easily worth triple that, and I could have used them almost immediately after my arrival. To add injury to insult, I had sold them right in the gamestore near uni I would be playing in for the coming 5 years (never again ran into the guy I sold them to either)...
So I decided to never again sell my minis, and I've held to this rule to this very day. It must be said though, that even though I have a very (un?)healthy hobby spending pattern, I never buy anything just for the heck of it. So every mini, piece of terrain or hobby supply is bought with at least a rudimentary plan in mind.
And that means that I am loathe to let go of anything I have bought so far. Granted; there have been collateral purchases (as in; I'll need 10 troops from a box, when there are 30 on the sprues) but, especially with all those bits services, Ebay and LAF, there are very few of those. And I usually trade those in for stuff I do have a need for.
So; unless I come to a point where I really, absolutely, undeniably cannot stash my purchases anywhere in my hobby room (or even garage) anymore, I will probably never sell anything. And if so, I gather I will probably have other problems to begin with...