Many people have a lot of stuff they no longer want or need, and some of it can be turned into cash on sites like eBay or Craig's List.
The most time-consuming part is probably doing research to find a reasonable price for the stuff. It won't be anything close to what you paid for it, probably more like 25-50% of the original cost, or even less. You'll also have to be aware of shipping costs, so stick to light objects that are easy to ship, or use a localized service like Craig's List for large items that local buyers can pick up themselves to avoid shipping.
If you like doing this, you can also provide it as a service for others in your area who don't have time or knowledge needed to do it themselves. You can do the price research, photography, and the posting. Add pick-up and mailing to your service, and keep a percentage of the sale price as a commission.