The question gets asked as though there is a right answer, and there is, but it is not the one either camp gives you. It depends entirely on what the figure is for, and most people have never actually decided that.
Start with the honest question
Not "will this be worth more sealed" but "what do I want from owning it". There are only really three answers. You want to look at it. You want to keep it as an object. Or you want to sell it one day. Each of those has a different correct action, and the arguments online are mostly people with different answers talking past each other.
What the box is actually worth
Boxed sells for more than loose. That is true and it is also the least interesting part, because the gap is not constant. It is largest on figures that were genuinely scarce and smallest on figures that were produced in quantity, which is to say the box adds most to what was already worth something, and very little to what was not.
The condition of the box matters nearly as much as whether it exists. A creased, sun-faded box on a shelf for six years is not the thing that commands a premium. Collectors buying sealed are buying an unopened, undamaged package, and most boxes on most shelves are neither.
When the box genuinely matters
Convention and store exclusives. The packaging is often the only thing identifying it as the exclusive. Open it and you frequently have an ordinary figure.
Short runs and numbered editions. Same reason: the number is usually printed on the package, not on the figure.
Anything you intend to have graded. Figures are graded in-box, sealed, and the grade is substantially a grade of the packaging. Opening it ends that option permanently.
When it does not
A mass-market figure from a line still in production is not an investment and treating it as one costs you the only thing it was definitely going to give you, which is the pleasure of owning it. The same is true of anything you have duplicates of, and of a figure you bought because you liked it rather than because it was scarce.
The middle paths nobody mentions
You can open carefully and keep the packaging flat. Most modern boxes can be opened along a glued seam and stored in a sleeve behind the display; the figure is out, the box is intact, and a future buyer gets both. It is not the same as sealed, and it is much better than a bin.
You can also buy two, which sounds glib and is what most serious collectors of a particular line actually do. One opened, one sealed, bought at the same time at the same price, and the decision is made once rather than agonised over per figure.
A straight answer
If the figure is an exclusive, numbered, or something you might grade: leave it sealed, and store the box properly rather than on a sunny shelf. If it is a figure from a current line that you bought because you wanted it: open it. A sealed figure you never look at, kept for a resale value that mostly is not there, is a box in a cupboard that you have chosen over the thing you wanted.
Frequently asked questions
Do sealed action figures really sell for more?
Usually, but the gap is widest on genuinely scarce figures and narrow on mass-market ones. Box condition matters nearly as much as whether the box exists.
Does opening a figure stop me having it graded?
Yes. Figures are graded sealed and in-box, and the grade is substantially a grade of the packaging. Opening ends that option permanently.
What if I want to display it and keep the value?
Open along the glued seam, keep the packaging flat in a sleeve, and store it out of sunlight. Not the same as sealed, far better than discarding it.
