Any item that you can buy for a little and sell for a lot will net you a profit. The secret is to have knowledge and access that other people don't. Some salable items are easier to find, while others are worth more money. If you can find them, the high-worth items will make you more money, for less work.
You can often double your money buying and selling furniture simply by having the means to move it around. There are plenty of people who'd buy that large bookcase you found at the auction, but they don't have a truck, and don't want to pay the added cost of having it moved. If you have a large vehicle, a strong back, and a spacious garage, you can take advantage of furniture deals. Resell bargain items to customers who'll pay more for the convenience of having the furniture delivered to their door.
Making money dealing in books is tricky, but it can be done. A primary rule is to be picky. Don't waste your time buying a book for a dollar and selling it for two dollars. Your time is better spent locating the books that actually have value. Buying a book for $100 and selling it online for $200 will make you 100 times the money, for the same amount of work. Condition is critical when dealing in old books, so don't buy anything without checking it over thoroughly. A good way to acquire valuable books is to buy them in lots at book auctions. Prices for individual books will go higher; the price per book will remain lower when the auctioneer is selling them by the box. If you buy the right box, you can do much more than double your money.
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If there's more than two of anything in the world, there's someone out there collecting it. From matchbooks to ceramic figurines to antique cars to barbed wire, the collecting bug bites everything. You can double, triple or quadruple your money by connecting objects to the people who collect them. The internet plays a huge part in this game, and there's a lot of competition. Be careful not to pay more for something than it's worth, and keep careful track of trends in the market. The collectibles market is notoriously fickle, and something that's selling on Ebay for $100 this year may be worth $20 next year.
If you're looking for higher profits, and willing to cope with higher risk, get into the big ticket items and start buying and selling houses. You'll need some serious capital to get into this game, but if you're smart and lucky, you can make more on a house in a year than you're likely to make on collectibles in a lifetime. Pay more attention to the location of a house than its condition. With enough time, labor and materials, any condition problems can be fixed, but location is permanent. Doubling your money in this field may take some time, but you might be happy with a 25 percent return -- if that means $25,000.