Sure Craiglist and eBay are great, but sometimes for one reason or another you need an alternative. There are some items that just aren't allowed on those sites--not even illegal or unseemly stuff either; dogs, cats and other pets for example. Everyone knows that buying used is the number one best way to save your money, especially on durable goods that don't wear out, so here are some places to find that stuff.
Kijiji (now known as eBay Classifieds) - I honestly don't know why this site exists. Since eBay owns a piece of Craiglist, and eBay also has fixed price listings now, eBay Classifieds is some kind of little brother to the big sites. They do have quite a few listings though because it is free to list. They are mostly local and they do allow pets.

Sell.com - Besides the catchy domain name this site has nothing. When a site doesn't even tell you how many results there are in a category, they are hiding something.

Skepter - "it's craigslist alternative" says the home page. Well they want to be, but they have a ways to go. For one thing they ought to hire a proof-reader and change their slogan to "It's the Craigslist Alternative" or "An Alternative to Craigslist." They do allow you to search the entire country from one page though, which Craigslist specifically prevents you from doing. It's a good thing too, because they have six cars listed in all of Los Angeles.

Oodle - Here's a sign of a useful site; within 60 miles of my house they have thousands of cars listed. They may be exaggerating, but I there seemed to be plenty of things listed. It seems you can post directly from Facebook to Oodle, which makes it as easy as posting to Facebook. They also list pets.

Recycler - This used to be the actual paper classifieds here in southern California, but recently they put a lot of effort into a redesign of their online presence and are giving the big boys a run for their money. They allow listings for pets. They have thousands of cars listed in Southern California, though far fewer in other parts of the country.

Penny Saver - This is another old paper classified source that has gone digital. The quality and quantity of their ads are not as good as some, but they are nationwide. They also have pets listed.

Upillar - Never heard of them before I started to write this, but they seem to have a decent layout and hundreds of cars within 50 miles of my house. They also list pets.

OLX - Basically a clone of Craigslist but with searchability by distance and other things that CL lacks, like nationwide search from one site. And like most of the CL competitors they do list pets. Quite a few ads, but not as many as some of the others.