The 50% flash sale was last weekend from Friday till Tuesday extended till Wednesday. There is now no sale/promotion active.

Oh I missed it

When it's going to be active again?

There are promotional offers a few times a year but normally only 10% or 15% discount. This one was special, because of the 50 year Interrail anniversary. Maybe they'll repeat it later this year, who knows. They don't tell in advance.

I was wondering if I can sell mine, I have the 30-day pass for 335 euros and I can't cancel it

The passes were not exchangable which means you can´t change the traveler details means sell it to others. (Could be used otherwise as fraud by buying huge amount of 50% promotional passes and sell them later when the promotion is over for a price of 75% means still a win of 25%. To avoid that Promotional passes are usually not exchangeable)