I cannot speak for this seller, but I do know that resealing happens often. I know someone who bought a box of 1975 Topps minis, opened the packs & resealed them. Those resealed packs graded very high through a major grading service by the way. Luckily....this person has no illusions of selling the graded packs. They are displayed very nicely next to the Bretts, Younts, Aarons & Ryans he pulled from them.
On a more personal note....I would never buy these packs (graded or not) for the simple fact that it is too easy to reseal them with no evidence of such.