Fake cards get all the attention. Resealed boxes are the more common problem and the harder one to catch, because everything inside a resealed box is completely genuine. Someone has simply opened it, taken the cards worth taking, put filler back in, and wrapped it up again.
You pay full price for a box that's already had its value removed. And because the cards are real, none of the usual counterfeit tests will help you.
Why it happens
Sealed product is bought and sold on trust, at scale, by people who never open it. That's an obvious opportunity. Shrink wrap equipment costs very little, the film is generic, and the whole job takes a couple of minutes once you've done it a few times.
It's most worth doing on expensive product with a small number of very valuable chase cards. That's why you hear about it constantly with high-end English sets and rarely with cheap bulk product — the maths only works when the hits are worth more than the effort.
What to look at, in order
Weight is the single best test. A sealed box of a given product weighs what it weighs, every time, to within a gram or two. If you can find out the correct weight for that product — forums and collector communities track this — a kitchen scale will tell you more in ten seconds than an hour of squinting at the wrapping. A box that's light is missing cards. A box that's the right weight has, at minimum, the right amount of cardboard in it.
Look at the seams. Factory shrink wrap is applied by machine, so the seams sit in consistent places — usually one long seam on the base and neat, symmetrical folds at each end, like a wrapped present done by something that doesn't get bored. Hand-wrapped boxes have seams in odd places, folds that don't match end to end, and often a seam running across a face where there shouldn't be one.
Check the tightness and the finish. Factory film is drum-tight and slightly textured. Reseals are often either too loose — you can slide the film around the box — or too glossy and smooth, because a heat gun has over-melted it. Look for ripples, bubbles, cloudy patches and areas where the film has been shrunk twice.
Find the perforations and tear strips. Many products have a printed tear strip or perforated opening built into the wrap. If it's missing, misaligned, or if the film has clearly been cut somewhere other than the intended opening, that's your answer.
Look at the box itself under the wrap. Opening a box and reclosing it leaves marks. Check the glued flaps for lifted edges, dried glue residue, or a slightly furred surface where cardboard has torn and been pressed back down. Look at the corners — a box that's been opened, emptied and refilled often doesn't sit quite as square as it did.
If it's already open, check the packs. Packs should be uniformly weighted, uniformly sized, and sitting in a consistent arrangement. Resealed packs frequently show soft creases along the crimped edges, a slightly wider or narrower seal than the others, or foil that's dulled where it's been reheated. A pack that feels different in your hand from its neighbours is worth a second look.
The things that don't prove anything
A dented corner isn't evidence — boxes get knocked about in transit. Neither is a bit of dust under the film, or the wrap being slightly loose on a product that's been in a warm loft. Plenty of legitimate boxes look scruffy.
Equally, a beautifully wrapped box proves nothing on its own. A competent reseal looks better than a factory job, because a person is trying harder than a machine.
The real defence is where you buy it
All of the above is damage limitation. The actual fix is not buying loose boxes from people who won't tell you where they got them.
A box that's come out of a sealed case, from someone who bought the case, has had no opportunity to be tampered with. Every hand a box passes through between the factory and you is a hand that could have opened it. So the questions worth asking a seller are simple ones: did this come from a case, did you open the case, and where did the case come from?
Anyone who buys properly can answer that without hesitating. Anyone who gets vague about it is telling you something.
How we handle it
We buy full sealed cases direct from our suppliers rather than picking up loose boxes off marketplaces, and we don't open, weigh, search or reseal anything. Boxes go out exactly as they came out of the case.
If anything ever turns up looking wrong, tell us and we'll put it right.
You might also want to read are Chinese Pokémon cards fake? and Chinese vs Japanese vs English Pokémon cards.