This reverses a principle that could strengthen. (A) tells us that IF a place is a good source of used exercise machines, then it will offer some well-maintained machines at reasonable prices. But we’re trying to prove that a place is a good source of machines; we’re not trying to prove what else must be true IF we start off knowing that a place is a good source of machines.

(B) tells us that the best kind of used exercise machine is one designed for home use. This doesn’t help us prove that gyms and fitness centers, which use machines that aren’t designed for home, are a good place to buy used machines.

Helps build a bridge from the premises to the conclusion. We know gyms and fitness centers are places you can get well-maintained, used exercise machines at reasonable prices. (C), then, allows us to conclude that those places are a good place to buy used exercise machines.

(D) allows us to conclude that certain places are NOT good places to buy used exercise machines. But we’re trying to prove that gyms and fitness centers ARE good places to buy used exercise machines.

(E) tells us that if a place is a good source of used exercised machines, then it won’t sell poorly maintained exercise machines at high prices. But this doesn’t help us prove that a place IS a good place to buy used exercise machines. We want “good source of used exercise machines” to appear on the right side of an “if, then” rule.