alterations on a used dress

I think the only way it could save on alterations is if the previous owner was built exactly like you were (or very similarly). The only real advantage I would see was if the previous owner was the same height as you.

I would also recommend asking about areas where you know you are considerably bigger or smaller than most women your size (if there are any). For instance, I am large chested for my height and broad hipped. If I saw a dress that was taken in at the bust or hips, I would be too afraid to buy it.