I think part of the reason why Transformers can be often among the harder toys to find sealed is that it's primary appeal is its ability to transform. One cannot really marvel at the engineering and design of the toy without physically transforming it; simply looking at the photos of the robot and alt mode(s) just shows you the toy at point A and point B, but doesn't really show you how it got there. That's something that's more appreciated by actually experiencing it first hand by transforming the toy yourself.
I don't mean to say this as a criticism to sealed collectors, but I'm just saying that I think this may be a significant contributing factor as to why sealed Transformers (especially G1 TFs) are harder to find on the secondary market than some other toys, like say sealed Star Wars figure. You're not missing out on as much from keeping a 70s/80s Star Wars figure sealed compared to keeping a Transformer sealed. The main play pattern of Transformers is the transformation, or as Hasbro calls it "puzzle play" that so captures the imagination and fascination of children. So some kids who might be inclined to keep their Star Wars toys sealed, may be less likely to make the same decision for Transformers.
So I reckon that's what makes sealed Transformers a far greater rarity in the aftermarket compared to some other toy lines. Among the sealed collectors here, how many of you started sealed collecting right from the beginning as children, or is it something you decided to do later as adults? I personally never knew any kids who kept their toys sealed when I was a kid in the 80s. At most, I knew a few rare individuals who kept the boxes of a few select favourite toys. Like a mate of mine who loves Trypticon; got this toy as a kid and has kept it in box ... until last year when his mother tried to throw it out (that's right, she threw out a complete loose in box Trypticon! And the box got damaged by rain and weather by time he got home and salvaged it out of the bin! Needless to say he was NOT happy!

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