The more demand there is for something, the easier it is to sell.
Conclusion
Used cars less than 10 years old are generally easier to sell than are cars that are 10 or more years old.
Evidence
The more demand for something, the easier it is to sell. (the author must be assuming that there's more demand for cars under 10 yrs old than over 10 yrs old)
Junkyards have a lot of demand for cars under 10 years old, since those cars have modern parts that can be resold for money. Meanwhile, cars older than 10 years have older parts that junkyards don't want as much, because there's not as much of a market for that.
Evaluate
Given that we have this rule , and given that we know there is more demand from junkyards for newer cars than for older cars,
How can we still argue that cars older than 10 yrs are generally easier to sell?
Well, the author seems to be pretty boxed in here in terms of only considering selling to a junkyard. Couldn't we say that there's a lot of demand for my 12 year old Honda Civic? People know these cars still drive well and if I'm selling it for $2000, there's probably a family with an 18 year old kid that would love to buy this cheap, dependable used car as the kid's first car.
Goal
Since we can object to this argument by saying, , the author is assuming there ISN'T a lot of demand from people (non-junkyard people) who want to buy these older cars.