My research from looking this up on the Native Instruments Forum, is you're allowed to install a single license on on 2 computer. For instance I have my copy on Traktor on my desk-top, which I use on a daily basis and my laptop wihich I use for gigs.
A used copy of traktor can be transferred. I would suggest you contact the original owner and ask him contact Native Instruments to release his claim on the license, if he hasn't already done so.
The Traktor Edition that comes with the DDJ-t1 is the Pioneer Edition and yes it supports 4 decks, but has some limited functionaly when compared to Traktor Pro 2.03 which also supports 4 decks