Of course they're happy to let you take a sealed unit. Less work for them.
Don't anyhow say la, maybe a few years ago or if you gam cheng gao gao with the fellow serving you as I experienced once. I told him frankly that I was selling it off and he was nice enough not to unbox it.
One of the key reasons why they MUST unbox the unit is due to numerous cases a while back where the phone was imperfect i.e. damaged or scratched. SingTel suffered some losses due to this and they could not even trust their supply chain to deliver perfect units.
As such, they made it mandatory for Hello Shops to unbox all units and let the customer inspect until song song and sign off that all is good, so that there will not be any dispute or ill will moving forward.
From what I know after speaking to 2nd hand mobile shops, it's the same as they have also been hit with 'sealed' unit scams. To them they will also open up a sealed unit to confirm that the phone is legit for fear of being taken for a ride. Some ah tiongs previously put rocks and weights inside the box then reseal it nicely before selling to dealers including to the one I spoke to, causing them to make a massive loss. After that they always open to check so I'm not sure if they will offer more for sealed units anymore.
Selling to individual buyers may net you that premium but they are taking that risk and I reckon the premium pricing for sealed units may be somewhat affected after these scams.