To me, a used ukulele is someone's cast off. Even if it is in pristine condition. I know that a lot of people here buy them, play them once or twice, and then get rid of them to make room for the next one. My logic tells me that there is a deal there, but in my gut tells me that it is the one that they didn't want. It is the one they were most willing to get rid of.
I think I get the idea of what you're saying, but I think it might depend on why someone is selling the ukulele.
For example, I'm toying with the idea of selling a ukulele that is a great ukulele, (IMO), but I'm realizing that I play better on smaller ukuleles, and can play longer on smaller ukuleles. (Might eventually sell the tenor, but keep Ohta San, soprano, and concert.) If I sell it, it's not really a cast off, but one that isn't right for me.
Some people sell because they've lost interest in ukulele, or have a health or financial situation that makes it necessary to stop playing.
Some people here on UU seem to like to experiment with different ukuleles, and try one out, and then pass it along so they can try something else. Their "cast off" might be a fantastic ukulele in perfect condition.
And, yes, some are probably less desirable, but we all differ in taste, and those cast offs might be perfect for someone else.
I did buy one used ukulele from the Market Place here. It was just what I was wanting, (Kinnard soprano), including the woods I wanted. The advantage of buying used was I got it right away instead of a long wait on the Kinnard build list, and got it for a great price. I love it.
There is risk in buying used, but I think that there a number of people here on UU who can be trusted to give an accurate description, and make it a painless process. When I was buying the used soprano, I went back through the person's posts, and felt like I could trust the person. (And fortunately, that was accurate.)
There is also risk in buying new, as the OP seems to have experienced with your unfortunate experiences with the Kala.
While buying from HMS, Mim, or Uke Republic can help you make sure that the ukulele is well set up, many of the ukuleles for sale here have been purchased from those places, and are well set up also.
We all have our personal preferences, and Rollie is very entitled to prefer to buy new, (I feel the same about some other items). But you might find a good price for a well loved and cared for ukulele if you buy used, so it will be up to the OP to decide what is comfortable for him/her. It might be one that someone regretfully is parting with, (wrong uke for the person, or circumstances requiring selling), but it might be the perfect ukulele for you.
Edited to add:
There are some stores who sell used ukuleles as well. Someone mentioned Elderly, and I personally wouldn't hesitate to buy one from them.
I recently contacted Acoustic Music Works about a used ukulele, and they basically told me that it's a lovely ukulele, but based on what I told them, they suggested that it wasn't right for me, so I think I would trust them as well, since they gave up a potential sale, preferring to have a happy customer.
There are other places as well, depending on your budget, (I think Ukulele Friend is mostly high end ukuleles, correct?)
Lots of options, and trustworthy places to get a good price on a potentially perfect-for-you ukulele.