Do anyone know how can Smoker hold Sea Stone when he himself a devil fruit user?
To expand on the current answers a bit, seastone is a mysterious substance that apparently has the same "wavelength as the sea" which we famously know to be the weakness of any devil fruit user, sapping them of their energy, powers and costing their ability to swim. The minutia of the effects of seastone are somewhat vague but a few things are quite concrete. One must be in direct contact with the seastone for it to have an effect, Luffy loses his abilities and becomes very tired when smoker pins him with the end of his jitte which is where the seastone is, the rest of the jitte is safe to the touch. Another example is that marine warship hulls are covered in seastones to ward off neptunians but people with devil fruit powers are unaffected by being in such close proximity to so much seastone such as when Hancock and Luffy rode one to Impel down. Seastone has varying levels of effectiveness which might have to do with how much of it is present, note the marines have seastone handcuffs which sap devil fruit users of their powers but doesn't seem to apply the strong lethargy something more substantial like Smoker's jitte does.