* Founder-CEO shares worth at least 350 mln euros
* Existing shareholders to sell 14.2 pct for 947 mln euros
* Company forecasts sales growth of 40 pct in 2018
* Final pricing expected June 12, listing June 13 (Updates with details; background. Books covered within an hour of launch)
By Toby Sterling
AMSTERDAM, June 5 (Reuters) - The listing of Adyen, the Dutch firm whose payments processing technology is used by Facebook and Netflix, got off to a strong start on Tuesday when investor demand for its shares covered those on offer within an hour.
Existing shareholders in Adyen, whose customers also include Uber and eBay, are selling a stake of up to 14.2 percent in the financial technology company which indicated in its prospectus its total equity would be valued at 6.5 billion-7.1 billion euros ($7.3 billion-$8.3 billion) when it lists on June 13.
Adyen is debt-free and profitable, and its listing, in the form of a secondary share offering, had been seen as one of the most prominent in Europe this year, after several were shelved last month.
Payments companies are in high demand.
U.S. giant PayPal, agreed to buy smartphone payment terminal provider iZettle last month for $2.2 billion in the midst of the Swedish firm's own IPO.
Adyen forecasts medium term net revenue growth of 25-30 percent per year, and of at least 40 percent in 2018. That follows a 2017 net revenue rise of 38 percent to 218 million euros.
"We feel that we are still in the early stages," said CEO Pieter van der Does, who holds shares worth at least 350 million euros, given the company's indicated price range of 220-240 euros per share.
"This offering provides us with the freedom to keep building the company, while offering our shareholders a path to liquidity."
The offer will be entirely secondary, the company's prospectus showed.
"Books are covered on the full deal size throughout the price range," the bookrunners said in a statement.
Current Adyen shareholders include Index Ventures, Felicis Ventures, Temasek and Iconiq Capital, the Silicon Valley fund that is an investment vehicle for the founders of Facebook, LinkedIn and Twitter.
Among major risk factors in the prospectus, Adyen cited fierce competition and the fact that its top 10 clients, which also include Vodafone and Spotify, represent around 33 percent of sales.
Adyen's many rivals include U.S. card processing company Vantiv, which bought a controlling stake in Britain's WorldPay in a $10 billion deal last year..
The prospectus confirmed a report that eBay was offered warrants worth up to a 5 percent stake in Adyen as part of the January 31 deal in which Adyen displaced former eBay subsidiary PayPal to become eBay's primary payments processor.
"Adyen considers this agreement to be of an exceptional nature and does not...expect to conclude similar agreements in the short term," it said.
Adyen published first quarter earnings along with the prospectus that showed it continuing the rapid growth it has enjoyed since its incorporation in 2006.
Net revenue rose by 67 percent to 74.4 million euros from the same period a year earlier, while net profit rose to 24.1 million euros from 14.1 million euros.
Reporting by Toby Sterling; Editing by Amrutha Gayathri and Alexander Smith