Nothing stops you. It is in a mylar, so resealing this might be a bit more visible. Basically something like this just makes folks feel more comfortable with the grade and resto. Same seller reputation, same apparent scans, whose book would a buyer have more confidence in? It is a $10 or $15 service depending on the book, so folks buying will feel more comfortable taking the book out.
Of course, the problem is, I might feel like a certain ebayer is not a scammer who would do a switcheroo, but how do I know if they bought the book from someone who is a scammer?