THIS is why you should ALWAYS charge at least a minimal re-homing fee for any animal. If they are willing to give you 20 bucks for it, they probably don't have something like this in mind. This cat person sounds like a dog fighter; giving cats to her dogs to kill to "boost their fighting spirit". Hoarders don't usually go out and look at ads for animals. They pick them up on the street, then they have babies, etc.
I saw an ad today that an FFA student posted, they wanted $20 for a mixed breed meat chicken, not sure what most meat chickens go for but that seemed steep!