thx for all the input
for udate: the story was a little more complicated than I thought at first. The person was offering me the domain sex.lu while he did not really own it, but rather more the domain does still show as registerable. What he in fact wanted to offer me - what I realised only later - was that he would be the admin-c for the domain, since you need to have a luxembourg citizen for admin-c in order to register .lu domains.
So basically I took him for his offer, but when I messaged a .lu registrar I got told, that the nic of Luxemburg would not allow anyone to register sex.lu
so basically I did not get it... itīs a pity