This can be achieved accumulatively and doesn't have to be done in one session. (Must be in the same save though)
Starting Tuesday the Cash 4 Cats vendor will open up outside the Wipe House. Simply walk up to the vendor and press E to sell him your pussy cats when prompted. He will give you cash in return.
You will have to use this vendor quite a lot if you're going for these achievements as well:
I wasn't able to sell 30 cats until Wednesday. When you see a cat DO NOT run at it or you will scare it off. You have to slow walk (Shift) towards the cat to pick it up.