In a tight economy, people are constantly looking for ways to save money, spend smarter, and earn a few extra dollars on the side. One of my friends, Eric Johnson, a technical trainer and supervisor in Des Moines, Iowa, uses several smartphone apps to make extra money by buying and selling merchandise online.

Eric is a savvy shopper, and he uses several smartphone apps to keep up with the latest deals (and sell merchandise to other people). Just like eBay used to be a popular way for people to make money by buying and selling merchandise, these smartphone apps are offering more efficient ways for people to find what they need – and for other people to earn a profit in the process.

“I’m a big fan of finding the best price on an item, and practicing arbitrage” (buying things low and then selling them high) said Eric. “I find that this combination of apps allows me to do quite a bit of both effectively with a minimal amount of time.”

Here are the smartphone apps that Eric Johnson uses to save money while shopping, and even earn some extra money in his spare time:

• Amazon app: Because he is a member of Amazon Prime, Eric gets free 2-day shipping on most purchases. The 2-day shipping makes it easy for him to quickly buy, sell and ship merchandise, whether it’s something he’s buying for himself, or something he’s buying and then re-selling to someone else. (We’ve written another article about Amazon and how you can use it to check prices and get up-to-date customer reviews.)
• Craigslist: The famous online classified website has a smartphone app as well. “Craigslist is the best local deals app, depending on where you live,” said Eric. The advantage of the Craigslist app is that it has a higher volume of users and most people recognize it and know how to use it.
• Eggdrop: This is another local deals app, showing what people want to buy and sell in your immediate area. “Eggdrop is a better, more fluid version of the Craigslist app,” said Eric. “Although there aren’t as many local people in my city on this app right now, I think it will take off in the future.”
• ShopSavvy: “This app has the best barcode scanner I’ve found for price comparisons,” said Eric.
• Square: This is an app that allows you to take credit card payments on your smartphone. If your friend wants to repay you for a meal you bought, or pay you for some used merchandise you’re selling to them, you can swipe their credit card on your smartphone. Eric uses his Square account to get credit card information from buyers online – he serves as an online “merchant,” buying and selling goods for a profit.

Eric said that all of these apps are simple and easy to use (“With the glaring exception of the Craigslist app,” he said). They all take up a small amount of memory on the phone, and they all can be pushed to the phone’s SD Card as well.

Eric has turned his interest in smartphone apps (and his nose for deals) into a fun and profitable side business. “There are times where I’ve been able to buy brand new merchandise at a deep discount and then immediately post and then sell the item on Eggdrop, Craigslist, or my Amazon storefront for a significant markup,” said Eric. “My Square account allows me to take credit card information to make these transactions easier for the buyer.”

Can anyone use these apps to make money by buying and selling merchandise online? Eric says that it’s still possible to find great deals (and sell the same items at a profit). And even though it’s not as easy as it used to be to find low-hanging fruit, there is an emerging market for secondary merchandise where people can buy and sell from each other, beyond the retail stores.

“I’ve found it more and more difficult to drive down the price of items,” said Eric. “More retailers now have ‘lowest advertised price’ settings to keep from being undercut constantly. I do think that as more and more people move to smartphones, there will be a larger market of secondary merchandise available. When you have millions of people who are able to take a photo from a smartphone, take payment via Square, PayPal or Dwolla for minimal interchange fees, and immediately post an item for sale, it’s going to make this market much more interesting. People won’t need to go to garage sales anymore. You can use your smartphone to do garage sale shopping from your couch.”

Does this article make you want to learn more about buying and selling merchandise online? You can follow Eric Johnson on Twitter.