Question1: how can I trust the vendor that he will accept the transaction once I send the money?
As BioMike said, if you can prove that you've sent the money, we can act on this. Another thing is that we're planning some other features that will make this easier and safer, but these things take time, as we not only need to code them, but also make sure that it complies with law in Poland.
Would it be possible to add all SEPA currencies, or is that a complicated process?
I would like to offer some coins in CHF and NOK would be good-
It isn't complicated, but we would need to see that there is actual demand for trades in these currencies. Then we could consider adding them.
Yes, I have some NOK that I'm looking to convert into BTC, and the seller on the otc that's offering BTC for NOK has a gpg key that won't work to encrypt messages.