I went over the paypal api docs and was bombarded with dependencies, prerequirements and stuff... one of those "oh this needs a new NET oh new NET needs a new OS oh new OS needs a new Computer" things..

So I may use the paypal html interface, by a form or even over the url. The "return" variable I can set to a path the payer will be redirected to after successful payment. This cannot be hidden totally, because it's in his browser, a fradulent person could call this url manually, making me think he has paid. There is ipv instant payment verification, an other paypal feature, seemingly harder to implement
...edit: IPN, not IPV:https://developer.paypal.com/docs/classic/ipn/integration-guide/IPNIntro/
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So, what is missing imho in the paypal html interface, is an encryption option for the "return" url,  the seller gives paypal the key and he gives the payer the encrypted url, that the payer will send to paypal, so they can send the payer the plain text url after payment. This url however should contain further security features, such as encrypted timestamp, uniqueness etc. to make this a one time green light thing.

However, if in an assumed poker game the bank hast to pay cash to a player, it still can/must be verified, whether he really paid the entrance fee.

Great about html5 is also that you are already in a browser, btw...