It may sound impossible but we funded our Disney Cruise for our family of 6 this year entirely by selling items around out house we were no longer using. It takes time and some work but all and all we made about $3400 over 18 months by selling things in our house we were no longer using!
I am personally not a fan of holding garage sales. I think they are a ton of work and don't always generate a significant amount of money. I have had garage sales that make $500-$700 if I have a lot of big ticket items. I have also had garage sales that barely make over $100. It is a lot of work and time for $100.
Craigslist has been a game changer in the buying and selling of used items. My large netting garage sales were held prior to Craigslist being a common forum to buy and sell used items. I remember using Craigslist as early as 2006. In the last 10 years I have done 100's of transactions, both buying and selling and have never ran into any problems. You have to use common sense and be safe but almost everyone else is using it to either save or make money, just like you.
Mattress I sold for $20 (1/4 retail)
If you've never used Craigslist the idea of meeting someone you don't know may seem weird. For this reason and due to recent press people often meet in public. I personally find this inconvenient and do most transactions on my front porch. I only invite people into my home if the item is too large or heavy to bring to my front door and in that case my 2nd Amendment loving husband is home. I also use the same caution when picking up an item at someone else's home. I wait on the front porch and bring someone with me.
Craigslist prices are similar to garage sales. Don't expect to get top dollar unless you have a rare, expensive, or highly coveted item. Even then, most things are only worth 25%-50% of original value unless it is a special exception. You can get an idea of what your item may sell for my looking at Craigslist in your area and surrounding area. The biggest tip I can give regarding pricing is don't overprice items. If you price your item right it will sell within in a day or two. If you price it too low you'll get multiple offers right away. If you don't get any inquiries within a day or two you likely have priced your item too high or just have an item that isn't likely to sell on Craigslist.
Pack N Play I sold for $50 (1/2 retail)
The items I have found that rarely sell on Craigslist are clothes and shoes. If you have seasonal or sport specific item that retails for $20 or more, it may be worth listing and see if you get any interest. Kids soccer cleats, kids snow boots, kids winter coats, etc. seem to have a good market on Craigslist. General clothes and shoes rarely sell. Occasionally a large lot can be sold for very little money. I have not found a time effective way to sell clothes or shoes so I either give hand-me-downs to friends or donate to Goodwill, take the tax receipt and write off the donation on my taxes.
Bed that was part of a bedroom set we sold for $200 (1/5 retail but was 10 years old)
Another forum I use to sell used items is Facebook buy/sell/trade or garage sale groups. You have to be a little picky about which ones you use. When I first heard about these groups a couple years back I was added to several but found that most weren't active enough to reliably sell items. I have used one local FB online garage sale group with 9,000 members to sell quite a bit the past 2 years. With that many members things tend to sell quickly. With my smart phone camera and the easy to use Facebook interface I prefer posting things to that group first and if they don't sell in a day or two I will also post to Craigslist.
Limited edition print cloth diapers I sold
Know your target audience. For specialty or kids items I targeted local groups I was a part of. I sold all of my cloth diapers through a local Facebook cloth diaper group I am a part of. I posted big ticket baby items to local moms groups I am a part of on Facebook before posting elsewhere. If I can sell to an acquaintance or friend who is looking for that item, even better.
Glider rocking chair I paid $50 used for and sold for $50 used 3 years later
I wasn't systematic about going through my house and purging. I just kept my eyes out for things we were no longer using or could live without. Once we had our cruise booked I knew I had 18 months to come up with the money so I started selling items 4-5 at a time. I found it a lot less overwhelming than having tons of stuff for sale at once. Once those items would sell (quickly if I priced them right) I'd then gather another 4-5 things, photograph and list the items for sale.
Another thing I did was I kept the money separate. I think this is very important if you are saving for a specific goal like a vacation. All the money I made from selling items I put into a separate vacation savings account. A lot of things I sold were in the $40-$50 range. Some were only $10-$20. We had a number of big ticket items like our playground, a bedroom set, and a motorcycle. My criteria about whether we should sell it is, were we using it and would be miss it? If the answer was no the item was sold.
Dog crate sold $50 (1/2 of retail)
Playground set sold $175 (retail $600, I paid $100 used)
The process is ongoing. I still list items for sale as I realize we are no longer using them. Just get started and be realistic about what your items are worth. Take pictures in good light (your smart phone will work), list items, and respond to inquiries promptly. $3400 in 18 months is a lot to make but we had several big ticket items: motorcycle, 1 full bedroom set, bed, playground, lots of baby gear, guitars, patio set, BBQ grill, and tons more. The other thing you'll find is not only will you feel accomplished by getting rid of stuff and making money, your house will feel less cluttered and cleaner. Buying used is a long time hobby of ours as well. We try to look for items 2nd hand before buying new. If you buy things inexpensively 2nd hand you can usually sell them several years later for what you paid.