Some of my first guns have been sold simply because either I wasn't using them or I upped the quality in general. My first good handgun was a 4" Colt Diamondback in 22LR. X-wife borrowed it after we split up and kept it... learned a lesson on that one. Of what I still own, I think my Mossberg 22 rifle is the oldest owned and probably a Model 700 .270 is next. A High Standard Victor is probably the oldest handgun I own these days in terms of my ownership. As far as why goes, the purpose was always to use initially.
I started attending gun shows in the 80's and the buying and selling gets into your blood. I thought, if they are doing it and making money, why not me? I mean, why else would they be doing it? I did that.... but never really made any money to speak of. Certainly covered the cost of admission at shows with the sales. But most importantly, it was fun. Not really interested any more in such activities although if I attend a show in state, I often will bring along something to sell or trade if the right opportunity presents itself. Basically, I culled out the ones I really didn't want or had little affection for and the remaining ones were keepers for me. Now I am starting to look at selling a few things again after a 15 year lapse in the activity.
I considered myself a collector for a while and I guess I still am, but most of the good ones are starting to have little appeal to keep and it's time to pass them on to the next generation of collectors.