Depends on the conditions under the pass was bought. Promotionalpasses usually not as they are not exchangeable / refundable :)

Otherwise i could buy under the 50% Promotion hundres of Passes and sell them later for the price of 75% Still 25% profit.

Hi! thank you for your answer, I just wanted to resell it at the buy price not to make a profit :)

You can't sell it to another person.

Depending on the conditions of the pass you can get a refund from Interrail. That is not possible for the passes from the 50% sale though. 

Resell is also not possible, because your name is on the Pass. 

Yes but how can Eurail know that. Because of that Promotional Passes are usually not refundable or exchangeable (except the Plus upgrade was bought) But for the 50% promopasses you couldn´t buy the Plus upgrade.