GREENSBORO, NC - We do a lot of stuff online, including buying and selling the stuff we don't want or need anymore. The extra money is always welcome, right? And of course, you want to make sure when you sell something and you receive money in exchange, the money you get is legit.

We found a $7 dollar item that can help you do just that. You ready? It's a...counterfeit pen. Okay, I know. You're thinking, really? I clicked on this for you to tell me about a counterfeit pen? Here's the bottom line. If you're going to be interacting with people you don't know and exchanging money, especially cash, a counterfeit pen can help protect you.

Recently, a viewer contacted 2 Wants to Know, saying he received $500 for two phones he sold. The viewer claims the money was counterfeit. Full transparency, we have not been able to get back in touch with viewer to verify his claim. However, a Google search shows dozens of people across the country have had the same problem.

The viewer says he used the letgo app, a mobile marketplace that lets people buy and sell things. On the letgo website, the company warns users "when receiving payment in cash, verify that the bills you receive aren't counterfeit."

The company also states very plainly, "the interaction and negotiations between buyers and sellers are the sole responsibility of the parties involved. Because of this, letgo cannot provide refunds or assist with disputes."

So, it's up to you to protect yourself. The pen is very simple to use. Simply swipe it across the bill. A yellow or colorless mark means the bill is real. A brown, black or deep red mark means the bill is fake.

It's important to note, the pen isn't fail safe. Criminals are smart and sometimes the pen can give a false positive. But, it's handy tool to help keep you safe. If you want more ways to spot fake bills, click here.