I can understand this completely, and it is a smart & safe policy.
But I just want to mention that I did buy my full version copy of Cubase 6.5 on ebay this summer, as well as a number of us who’ve jumped ship from Cake’s Sonar.
I too was leery of it, so I just did my homework first. 
I contacted a buyer who bought from the same guy just a few days before, and asked if it was legit, and also asked if Steinberg recognized the serial number & authorized it.
I also saw that a number of other people were buying the same item, increasingly from 2 a day up to about 4-5 a day.
So I took a chance on it, and it was all good.
I did pay through Paypal though, as always, to be safe. 