Adoption Agency
One way to make money, though not guaranteed, is to adopt expensive pets from the adoption agency.
http://www.neopets.com/pound.phtml
You can check to see if they have expensive battle equipment, or a petpet. Be careful, cause it could cost a few thousand to adopt and send back the pet after you take what you can.
You can also hold the pet for a day or two, to see if the owner will give you money to "give back" the pet. I haven't done this too many times, cause usually there isn't much battle equipment or petpets on the pets. The more expensive it is to adopt a pet, the higher its levels, so that's why I target those. Plus those are ones that the original owner is more likely to want back.