You might be surprised to hear that more than 450 million people visit buy-and-sell groups on Facebook each month - nearly three times as many as use eBay.
And making a bit of extra cash on the site is set to become even easier, with the launch of a new service called Facebook Marketplace earlier this month.
Marketplace is rather like Craigslist, in that it's simply a place where buyers and sellers can come together. It doesn't offer any protection to buyers or sellers or take a cut of the proceeds, as eBay does.
However, unlike Craigslist, it displays the location of both parties, along with public Facebook profiles and cover photos which means that you should be able to have a little more confidence about who you're dealing with.
To sell an item, you simply post a photo, along with a product name, description and price, as well as confirming your location and selecting a category. Potential buyers will message you, and you can discuss price, condition and how the sale will actually be made.
"Marketplace makes it easy to find new things you'll love, and find a new home for the things you're ready to part with," says director of product management Mary Ku.
When setting a price, take a look at what people have charged before - on eBay, if you can't find a Marketplace listing. And unless you're in a screaming hurry, you might as well set the price on the high side, as the site includes a 'Make Offer' button.
You may also want to take a look at your Facebook profile, as buyers will be able to see it. If your profile picture is of you drunk and in fancy dress, you won't necessarily look all that reputable.
Although there have been a few teething troubles, with items such as drugs and black market cigarettes turning up on the site, listings on Facebook Marketplace are vetted by the company.
But not all opportunities for selling on Facebook are all that they seem.
The chances are that at some point a Facebook friend of yours has started raving about a particular product: a nutritional milkshake, a brand of cosmetics, a herbal supplement.
This friend may tell you that they're earning a fortune from sales, and encourage you to sign up yourself so that you too can afford a Mercedes while working just a few hours a week.
You'll own your own business, they say, and be more available for your children.
However, such so-called Multi-Level Marketing (MLM) businesses are nothing but pyramid schemes - and there's no chance at all of your ever being able to afford that car. In fact the odds are that you'll end up significantly poorer.
The first person in such a scheme sells their product - but even so, earns very little money from sales. Where they do have a chance of making money is by signing up their friends to be salespeople too, and taking a commission.
They'll also sell their friends a 'business starter pack', often costing hundreds of pounds.
However, unless you're one of the very first people to sign up for one of these schemes, your chances of finding customers are slim. In 2014, according to the Direct Selling Association, there were more than 18 million people trying to sell 'miracle' products this way.
The odds are that the person that signed you up has already approached most of your friends; and if they haven't managed to get them all to sign up, the chances are that you won't either.
And that's before you've taken into account the cost of getting involved in the first place. In Canada, MLM companies are obliged to reveal how much their salespeople make on average - and it's less than £1,000 a year.
However, most MLM schemes require salespeople to buy samples, sign up for paid-for training courses and perhaps contribute to website costs, meaning this £1,000 can be wiped out completely.
Finally, of course, you'll really annoy all your friends.
Earlier this year, the US Federal Trade Commission (FTC) took action against MLM company Herbalife, ordering it to pay back $200 million to people who had fallen for its claims.
"Advertising in English and Spanish, Herbalife pitched its business opportunity as a way for people to quit their jobs and make the big bucks. Other ads promoted Herbalife as a means for already hard-working people to provide a little more for their families: 'When we worked in factories our earnings could only pay for basic needs, but now we can take our 12 grandkids on vacations'," says the FTC's Lesley Fair.
"But don't start packing the kids' bags because according to the FTC, it's virtually impossible to make money selling Herbalife products... our analysis shows that half of Herbalife 'Sales Leaders' earned on average less than $5 a month from product sales."