When I first started collecting mechanical puzzles (and before I started making them) I used to browse Ebay, always on the lookout for interlocking puzzles, burr puzzles and puzzle boxes. In one case I found a nice Russian puzzle box with a concealed drawer which I simply couldn't figure out how to open. I tried for days, gave it to my friends to try...nothing worked. No progress whatsoever.
So I emailed the seller, a long-time collector who is now a customer of mine funnily enough. I told him I thought the box was broken, and could he provide me a solution. He replied that he had never solved it himself, and didn't know how to open it.
Frustrated, I kept trying and a couple of days later figured it out. I was ecstatic! I emailed the seller a picture of the opened drawer and explained the solution. His reply went something along the lines of "Good job! I knew how it worked all along, I just didn't want to spoil the solve for you".
That made a big impact on me. I'm so grateful he didn't spill the beans. When I started making boxes myself I remembered this interaction and vowed to follow the same philosophy.
It's obvious to me that half of what we sell is the physical puzzle itself...the other half is the “aha!” moment you feel when you solve it. With many puzzles there is an intuitive leap necessary to progress, and when a solution is given it cheats the solver out of making that connection themselves.
Therefore I’ve never included solutions with puzzles...the temptation to peek is too great. With many puzzles, particularly boxes, once you have key knowledge you can never "un-learn" it. That's why I do everything I can to avoid giving solutions to boxes for at least a few months after their release.