Not necessary, because even if the salability of something can be influenced by other factors besides demand, we still know from a premise that the more demand for something, the easier it is to sell. Demand for a thing can still play a dominant role in the ease of a sale, even if there are other factors involved. (Notice that the conclusion just says that cars under 10 years old are “generally” easier to sell; this doesn’t mean that they’re always easier to sell.)

Not necessary, because even if some used cars that are at least 10 years old are sold to places besides junkyards, the author just needs to assume that the demand of junkyards for those cars is reflective of the demand of other kinds of buyers, or that those other buyers don’t significantly impact overall demand for used cars.

Not necessary, because the argument is based on the relationship between demand and ease of selling. Although in this case the newer cars tend to be more in demand than older cars, that doesn’t commit the author to believing that this relationship holds with other items besides cars.

The argument concerns the ease of selling, not the price of cars. In any case, the argument assumes that cars that are less than 10 years old are in higher demand. So (D)’s reference to “lack of demand” for cars less than 10 years old doesn’t make sense.

Necessary, because if it were not true — if the salability of cars that are at least 10 years old is NOT largely a function of the level of demand for their parts — then the author would not be able to go from “little demand for car parts that are at least 10 years old” to “junkyards tend not to buy cars that are at least 10 years old.” The author’s overlooking the possibility that cars at least 10 years old might be in demand by junkyards for other reasons (perhaps they can be turned into scrap metal).