My husband tends to favor Amazon.com. Most of his items are books or electronics, so Amazon is great. No need to post a picture, just find your item and list it under the "used" section. You'll get an email if and when someone purchases it.
Unlike my husband, I find myself putting more household items and clothing into a resale pile. Mine are taken to Puttin' on the Ritz, a consignment store in Middleton. I drop things off once a season and usually receive a check for $25. Not bad for items I'd otherwise take to a thrift store.
In the past I used Ebay to sell items, specifically plastic toys from my childhood. I was amazed at how much people would pay for Barbie and other 1970s toys. The proceeds were put towards paying off my student loans! Ebay worked well for this items, they were no longer in production and needed a wide audience.
Finally there is Craigslist. I find this works best for furniture. Too big to ship. Easy to photograph. After buying our new home I sold the kitchen table that came with the house, it just wasn't for us. I snapped a photo, wrote a brief description, and within 36 hours a stranger arrived, paid me cash, and gave it a new life. The only concern I have about Craigslist is having a stranger come to the home, so it is nice to coordinate when others will be here.
So, if you are looking to be frugal take a look around. What items are never used, pile them up, and then ask - are they worth selling. Personally, it feels great keeping the clutter to a minimum in our home.