From music festivals to sold-out stadiums, big-ticket concerts can be one of the highlights of summer. But what if your tickets turn out to be fake. Millions of fans have been victims of counterfeit... See More
From music festivals to sold-out stadiums, big-ticket concerts can be one of the highlights of summer. But what if your tickets turn out to be fake. Millions of fans have been victims of counterfeit sellers. It's big business and happening across the country. Tonight, we're going undercover to see if we can catch them red-handed. Here's ABC's Mara schiavocampo. Reporter: From beyonce. To Justin Timberlake. ? Take back the night ? Reporter: To one direction. ? The story of my life ? Reporter: The hottest concerts of the summer are often the hardest and costliest tickets to come by. But sometimes the tickets aren't even real. Bruno Mars is one of the biggest acts on tour this summer. ? Love like a river ? Reporter: Dakota bought tickets to his sold-out show with a seller she met on craigslist. He said he could sell those tickets for $300 each. Reporter: When she got to the concert. They put a void on my e-ticket. And handed it back to me and said, sorry. These are fake tickets. Reporter: Fakes, forgeries. Across the country, counterfeit tickets are a real problem. How big is this problem? How widespread do you think it is? I think for any sold-out event, it's fairly widespread. And it's increased over the years. There's been some shows that you've seen anywhere from 4% to 10%, could be counterfeit. Reporter: Every year, nearly 5 million fans buy counterfeit concert tickets. "Nightline" embarked on an undercover investigation, with the help of ticketmaster, to discover how fake tickets are sold to you. Sorry I wouldn't call you back sooner. Reporter: Here's how it worked. An ABC news producer and intern went on craigslist, looking for tickets. I saw your listing on craigslist for the rihanna/eminem tickets. Reporter: They got the seller to send them a jpeg tickets. And sent the jpeg to ticket master. In one case, the seats didn't exist. We set up a buy with a seller we knew had fake tickets. So, the two tickets. $200 each. 7 and 8. Okay. Got her. For $400, right? Yeah. Okay. And I'm good. These are real? Yes. Everything is legit. I never bought anything off of craigslist. The thing is, I was supposed to go to the show with my girlfriend. They're not going to be in town until October. We're going to try to sell the tickets and try to get tickets to the October show. I see. Okay. Thank you for the tickets. Thank you. Nice to meet you. Enjoy. All right. Thanks. We've got the tickets. We have two tickets for Billy Joel in concert at Madison square garden. Yeah. We've got two tickets we can't do anything with. According to ticket master, these tickets aren't real. Reporter: We sat down with an executive to show her what we were able to find. Is this the way to buy tickets? You wouldn't buy anything. If you go on line and said, I'll meet you on the corner, would you do that for any other product? Reporter: We showed her the tickets we bought to see if he could tell they were fake. Do these look real to you? Yes. Those look real. Reporter: There's nothing here that the consumer would be able to spot to say, this is a red flag? Counterfeit tickets aren't unique to concerts. Where is it the worst? You're seeing it for all of the hot tours right now. Many big sporting events. And you're seeing it on Broadway, with another megahit. Reporter: The problem is so widespread, the L.A. County sheriff's department has formed a unit of volunteers to scour the web for red flags. It's a crime that, with computers and printers, relatively easy to pull off now. They believe their chances of getting caught are low. That's something we're going to change. Reporter: They shared one piece of advice for consumers. Only buy tickets through authorized dealers. You will not be able to tell the difference between the real tickets and the fakes. And you have no resource if you become a victim. Reporter: We wanted to confront one of the sellers out there trying to rip you off. So, once again, we set out in search of fake tickets. The two tickets, $350 total, right? Reporter: It wasn't hard. We confirmed with ticket master that these Billy Joel tickets are fake. Reporter: In a few hours, we arranged to buy two tickets to Billy Joel for $350. Hi. What's up? So, you got the tickets? Yes. And these are legit? Yes. Cool. 220, 40, 60, 80, 100. 20, 30, 40, 50. Thank you. Enjoy. Thank you. Have a good time. Thank you so much. Reporter: Excuse me. We're with ABC news. And we know that these tickets aren't real. Why are you selling people fake tickets? Why are you selling people fake tickets? We confirmed they're counterfeit. I don't know what you're talking about. Reporter: Do you have anything to say about the people who see this? Can we have our money back? If you promise you won't put this on the news. Reporter: After both buys, we confirmed again with ticket master that we had bought fake tickets. If you happen to be a victim of this, you buy a ticket with the best intentions, you spend your good money, you think it's real. You get to the venue and you find out it's not a real ticket, what can you do? Unfortunately, there's not much you can do. You should really read the fine print. When you're buying that ticket off of the site, at least you're getting your money back. But you won't get into the event. There is nothing you can do. Reporter: There's no recourse at that point. Correct. If you want a ticket and you want to make sure it's a real ticket, you have to go to the source. And if you don't find them, if it's on ticket master, we have created a way for people to buy a resold ticket and verify that it's a real ticket. Dakota was 1 of the 400 fans at the Bruno Mars show in los Angeles who did not get in. After finding out that the tickets were fraudulent and going through that embarrassment of having to walk away from the venue, it felt like a walk of shame. Reporter: For "Nightline," I'm Mara schiavocampo, in New York.
This transcript has been automatically generated and may not be 100% accurate.