ok, here it has been assumed that the plane is smooth ,therefore the only force acting on the particle is the component of its weight which is prallel to the plane(slope), and as you now the resultant force = mass * acceleration, so the resultant force in this case = mgsin(theta)-0( friction, which can simply be neglected!!)..therefore for an object moving down a smooth palne it will Always have an acceleration of mgsin(theta)/m....= gsin(theta), and thefore for an object moving up the plane its acc will be - gsin(theta).sorry for not being able to justify why is your method incorrect.
