While there are some sites out there, even some local to us here in the Ottawa Valley who want you to post only on their site and no one elses', we here at the Penny Pincher Newspaper want one thing for you: For you to sell your stuff.
This means being a realist; While we believe we have one of the more advanced and user-friendly classifieds sites out there on the 'net, we also know there are a lot of others that you post your items to: This is a good thing!
There are other classifieds websites, there are facebook groups, there are usenet news groups, there are bulletin boards in your malls, schools, community centres, grocery stores and local convenience stores - If you really want your items to sell, you should ideally have ads in all these places.
Naturally, the effort you're going to go through for ad exposure depends on the item and the price you're asking. For a baby's bib that you're asking $1.00 for, a few free websites are more than enough. Selling your '89 one ton truck, on the other hand, you'll probably want to spend the time, given you're probably asking a few thousand dollars
If you are newish to selling your stuff via classifieds, ask around your neighbourhood: Your friends, neighbours and colleagues: Where do they sell their stuff ? More importantly: Where do they look for stuff to buy ? It is in these places that you should be posting your ads for your items.
At the end of the day, selling your used items is a good thing: I'm no tree hugger, but still, if you can avoid putting things in a land fill, especially when someone else can get good use out of it and you can make a dollar off it - Hey, who's to complain ?