If I sell a used object that I don’t need anymore (eg. clothing), do I need to take off maaser from the amount I get paid?
It depends on how you got the object. If you paid for it yourself, then you most probably took off masser from the money that you used to buy the item, and then the money that will make back from the purchase was already “ma’asered”. (Similar to a store keeper that gives maser from the initial investment, and after that he only gives maser from the profits.) However if you got the object as a present, and you didn’t take off ma’aser from its value, then you should give maser from the money that you made will make from the sale.
Hatzlocho selling your things.