Tickets for Harry Potter And The Cursed Child were already up for resale for £600 while thousands of fans were still waiting in the online queue for tickets.
Fans of the wizarding franchise have been desperate to get their hands on the 250,000 tickets which went on sale at 11am on Thursday as the show extended its West End run until December 2017.
But while Twitter users shared screengrabs showing there were over 150,000 people left waiting in the queue, touts had already bought tickets and placed them for sale on sites including StubHub and eBay.
Tickets to parts one and two of the Harry Potter play in autumn 2017 are on offer for £600 and up on eBay, while some sellers on StubHub hope to fetch over £900.
Fans who were able to get hold of tickets from official vendors Nimax and ATG paid significantly less, with seats from £15 and 300 tickets for every performance priced at £20 or less for each of the two plays.
A Friday Forty lottery also gives hopefuls a chance to ballot for cheap tickets every week.
A spokesman for the show said: "Please note that we reserve the right to refuse admission to customers with tickets purchased on re-sale websites.
"Tickets purchased through either of our official ticketing platforms must not be sold or advertised for sale on the internet, in newspapers or elsewhere.
"Any ticket advertised for sale in this way will be automatically void."
Twitter user Caroline Diggens commented: "Failed to get #CursedChild tickets after queuing for 4 hours. Now loads on sale for hundreds of pounds on stubhub #fuming #shouldntbeallowed".
Holly Jordan wrote: "Still trying to get tickets for myself and they are already up on StubHub! Not fair #CursedChild".
Marlies van Eunen wrote: "Wizardingworld unite and let us all rise against the evil and foul souls selling their #CursedChild tickets on ebay. Wands out".
Asking for eBay to shut down the resales, Jennifer Rosellen wrote: "To the people selling #CursedChild tickets on @eBay_UK for £1,000+ you are literally vile scum."
Speaking on the red carpet for the production's gala opening on Saturday, author JK Rowling said she wanted to make the theatre accessible to people from all backgrounds.
She told reporters: "What we would really like most of all is to bring people in who have never been to the theatre before.
"I would be so proud to think that kids from my kind of background, who didn't come from particularly theatre-going families, learn what theatre is about through this show. That would be an incredible thing."