You shouldn't manipulate people.
Lily knows lots of personal secrets about Merlin and needs to get some work done on her car.
Lily wants Merlin to fix her car.
Lily offers to pay Merlin to work on her car.
Lily tells Merlin to work on her car or else she'll expose his secrets.
Merlin agrees and earns some extra money.
Merlin agrees but loses respect for Lily.