To assess the quality means that you are looking at the good and the bad to determine if the toy is a good value and worth the price or if it would be a good toy for the child. After you assess a toy, for instance, you may determine that the child is too young for that toy. To estimate means simply that you try to determine an approximate price or value of something. Or you can estimate (try to determine) size, number, height, weight, time, etc.