you have to be selective with what sets, and probably have to hold for a decent amount of time. It’s harder to find undervalued things in this type of market but can still be done. Wait for the attention to shift, or look other places.
Like most things, Pokemon operates on attention. You will make the most money buying etbs/boxes when no one is paying attention to it and products sit on the shelf (fusion, cosmic eclipse etc.) Right now attention is high, so yes you can make money holding products but ..