Well, you're an unknown newbie... 20€ beforehand, 280€ afterwards does indeed mean that the person taking your deal doesn't lose money, but it also means that the odds of you dissapearing and not paying the 280€ are great... No offence, but what would be your incentive to pay the 280€?
Maybe using an escrow would increase your chances of finding somebody to make the purchase?
By the way, i assume you want to buy something legal?
Yes it's legal, these are online services, available in play store/App Store.
In this case, he shows me the proof that he has bought well, without giving me the access,
I send half, and the other half when he gave me the access.
Seems fair to me.