I've talked to Quark about this a lot. We also needed to purchase a used copy. €1.999 is a bit out of our price range. Anyway, the gist of what they told me is that it would not be illegal to buy the used copy. In fact, they directed me to eBay and MercadoLibre. But, you would not be able to count on them for technical assistance.
There was some disagreement about whether I would be eligible for upgrades from them if I wasn't the original purchaser.
Still, as long you have the registration number and you're the only owner of the copy (that is to say that the original owner didn't try to get one over on them by keeping his/her own copy of the program and trying to upgrade), you should probably be OK. I don't think they will ask for your name to go along with the number.
All that said, even without technical help directly from Quark, it's definitely worth it. You can buy a used copy of Quark much cheaper. And in the end, who needs to upgrade. I just found out that our new service bureau only has Quark 3.3! Crazy