We've been concerned about the large numbers of animals being advertised on "Gumtree" and similar sites for a long time, and a call I took over the weekend gave me a very nasty insight into what can happen.
The caller wanted to find out whether the RSPCA could prosecute a man who bought a dog from him, and had the dog put down as dangerous within a few days of the transaction. Of course this wouldn't be possible because it's not illegal to have a vet put an animal to sleep—it's illegal to cause an animal to suffer, but not to kill.
If all the details of the story are correct, it appears that the purchaser changed his mind about wanting the dog, and possibly expected to be able to get his money back in order to buy a puppy (also, incidentally of a "status" breed). The seller couldn't have taken the dog back as his original reason for selling was friction with an existing dog (although he seems to have several other dogs and I wonder whether he is also breeding). He does appear to have left the purchaser a contact number when he sold the dog and offered to try to help solve any problems.
Background details of the various dogs involved are complicated, but there appears to be a whole ecosystem of people breeding dogs at home. Their motivation is probably a combination of the attraction of "cash in hand" income and a fairly genuine interest in dogs as a hobby. I wonder how much of it is driven by a desire to find a substitute for meaningful work that they can take pride in and whether an improvement in the job market would make things better.
An awful lot of dogs seem to be traded around like so many packets of sugar, with not much common sense about the presence of small children, or suitable facilities. Against this background our messages about not getting animals you can't afford to care for probably aren't going to make very much of an impact.
The people we're trying to persuade are being lectured all the time—don't drink so much, don't smoke, eat more fruit, lose weight ... it's not hard to see why they're not going to pay attention to us.