What if the seller opened, and re-closed them professionally? why not just open them before the sale to see what you are buying?
Is that a person you know, or one of the CL deals where you meet at a public space and will never know who the person was (you know, when it turns out to be just scrap metal in a box)
the story of " bought a bike but never rode it" is unbelievable. but the story " bought the bike, but never opened the box" even less so.
Good luck, you will need it.