You can start to sell the kittens when they can eat solid foods. If they are still drinking milk from the mom then you can not take them but when they eat solid foods then you can sell them.
I know what you are talking about b/c I saw that my cat had 5 kittens and 1 of them was so much smaller than the others but today I saw that that 1 was gone and there was only 4 kittens so I think it died so the mother eats them.
No I touch my baby kittens everyday and they are alway there when I get home from school.