Lots of reasons to buy/sell/trade:
1. Enjoy trying a variety of ukes, buy used what looks like a good opportunity on the marketplace, play it for a while and sell it. It's a way of "sharing".
2. Sometimes people buy a few ukes and after playing for a while decide that they really don't want or care for soprano/tenor/concert/baritone or whatever and they sell the one or ones of the size/s they don't want.
3. By chance I found some very nice used ukes locally. They were a good deal and nice ukes so I bought them. It left me with far too many ukes so I had to sell some off.
4. Sometimes I just don't "bond" with a particular instrument. Doesn't mean it isn't good or that someone else won't love it, just that it doesn't do it for me personally.
5. Sometimes I realize that I just play certain ukes more than others and I need to thin the herd.
6. OK some of us (like me) probably buy too many ukes. I never buy one I don't like or I wouldn't buy it, but some get played more than others.
7. Somtimes people have circumstances that change and they need money. Having a baby, losing a job, lots of things can make cash seem more desirable than a uke.