You may find a professional cleaning and new paint are required, possibly even new carpet and pad is needed if the pet was "accident" prone. You really CAN'T mask the odor --- you need to eradicate it.

We've been doing a lot of house hunting in the last few weeks and we won't even consider properties that smell bad; and the candles and other scented products they use to try to hide them just don't work. How anyone can live like that, let alone expect to try to sell it to others, is beyond me. I've heard from a real estate professional, with decades of experience in the business, say you will lose $10,000 right off the top if a home smells like pets. If there is also odor from smoking as well as pets, you can look at $15,000.