There are many websites which sell Movie Tickets or tickets to concerts or similar events. The problem however is scammers have also learned that they can make some easy money from those who don't know of the legit ticket selling sites. These sites have you pay for the ticket but after the payment is made the tickets are never delivered. If you can; avoid buying tickets online unless you know it's a legit site like Fandango or Movietickets.com. Avoid all the other smaller sites you've never heard of.
I never had an experience in buying movie tickets from online but I remember buying an F1 ticket last year with my husband and we bought it from it's official website. People should really avoid from spending money online unless they know that the site is real and not a scam.
That's why it's important to always buy only on the legitimate website that sells tickets, or on the official website itself, because there are many people who are gonna take advantage of unwitting customers who happen to stumble upon their site. If they have paid via Paypal, then maybe they can still get their money back.