Wondering if the "take away service" is bugged? My sim tested to sell a puppy, she got §350 (expected more, but ok), but the adoption person never left the lot. My sim could not interact with her either. I had to close my game without saving, which was what I intended to do anyway, but if selling pets don't work, I'll just not do it again...
Edit: Also, I expected to get more for a trained puppy than a newborn, but it seems that the earning is totally random? The 350-puppy even had learned to pee outside.
Edit2: Tried a few more times. Another puppy, much younger, also brought in §350,- so perhaps it's a new standard fee. The adoption person worked properly on the two new tests, I was able to save my game with no plunder, and the process was even rather effective.