Hi t.ruth,
Thank you for your concern on this issue, you have certainly raised a good question which we actually addressed with other members in detail in an old thread covered with dust over half a year ago. Anyway, I will address your question here again for the benefit of newer members.
Why Pets For Sale?
First of all, PetFinder strives to be the de facto community for animal lovers - and that includes rescuers, fosterers, adopters, breeders and buyers. Our mission is to properly educate the public on responsible pet management and increase their knowledge on their pets' welfare. Now, to achieve that, we need to bring in as diversified a community as possible, not just people who are interested in adopting pets which is a subset of the community.
With the Pets for Sale section, we will attract the interest of people looking to buy pets. Getting them here is half the battle won. Why? Because by coming here, there is a chance for them to be influenced by us to adopt these homeless animals, and as they also see our ongoing discussions on the benefits of doing so, they might actually change their minds. If you have checked out the profiles of pets for sale, you'd also have noticed that it also suggests similar pets that are available for adoption, so the person can actually see homeless pets with a huge difference in price, but equally cute and adorable.
Also, another mission of ours is the reduction of strays through neutering and spaying. This applies to all pets, purchased or adopted, so we must be able to attract all crowds of animal lovers and properly educate them. Even if someone ultimately decides to purchase a pet, he would likely have been exposed to our discussions on neutering his pet, and that would already make a difference to the overpopulation of strays.
What Happens If We Did Not Have The Section?
Let's face it, even if we did not have such a section, people who want to buy pets will still buy them from pet shops. Simply because pet shops are so easily accessible compared to animal shelters, and they are much more well-presented to attract customers.
Instead of letting these people purchase from pet shops which bypasses our channel of communication, we can now attract them over and have a chance to educate them. We do not have unlimited resources to reach out to these people at every corner of the street, so the next best thing is to make them come and reach in to us.
Chelly also made a good point earlier - if we did not have such a section, it is still not going to stop breeders from getting their pets published at our site. They would just end up classifying their pets as for adoption and fake the price, then when adopters call them up they will reveal their actual intentions. This would just end up diluting our genuine adoption listings, reducing adoption effectiveness and irritating the adopters. Now that we give them a proper outlet to sell pets, they are more likely to abide by the rules.
What About The Breeders?
Some people tend to stereotype breeders and pet sellers as bad, unethical, cruel etc... but that is not necessary the case, as we can see from the excellent example of Jimmy (Coony). What we need is to have responsible breeders like him to be around, and to help lead the way for others.
There might eventually be ignorant breeders that slowly see our points and learn from them. Also, customers that are now more educated would demand a better practice from the breeders or ask more educated questions, which would lead to the breeders adopting higher standards.
For the stubborn and unethical ones, they stand to be exposed here by our members. That is the reason we have a public forum, hall of shame and comments sections for people to leave feedback. If someone is found to be ill-treating animals, we will unreservedly expose them to the public. With a feedback channel in place, it makes this process more organized and transparent, instead of letting these people lurk in the shadows and escape our radar.
Hope this sufficiently addresses your concerns. Feel free to let me know if you have any other questions.