Recently Just bought a DDJ-T1 from a pawn shop. how do i get the software key?

I'm not sure how you'd get the licence key for this as it's actually a Pioneer product rather than Native Instruments so the usual way of getting the licence described here probably won't work - https://support.native-instruments.com/hc/en-us/articles/360001301669-My-Serial-Is-Already-Registered-to-Another-User-Account

You can try putting in a support request as described in the link and see what they say. Maybe @Jeremy_NI has a better idea? Bear in mind support don't work over the weekend so you might not get a response until Monday.

With respect to the software then I think that you are basically in the same situation as if you bought some second hand software of the internet . You would have to get the original owner of the Pioneer device to transfer the serial to you ! Ref. : Buying or Selling a Native Instruments Product Second-Hand , and you obviously can not get the original owner of the Pioneer device to do that. Else then I don't know if N.I. is giving away free licenses of old software. (in this case Traktor Pro 2) But you can get the FREE TRAKTOR DJ 2 here ! And you can buy TRAKTOR PRO 3 here ! (EDIT : The free Traktor DJ 2 might not work , reference post by Lord-Carlos here !)

The TRAKTOR PIONEER DDJ-T1 edition delivered with the DDJ-T1 was a limited edition, meaning it only has limited functionality but it were also a "customized" version of Traktor software. Also the Traktor version might have been depending on generation of Pioneer product : Reference !

Other information :

Reference Pioneer support ! : I lost the serial key to activate TRAKTOR software. Can I get the key reissued? (NO !)

Anyway , no matter what then you will probably need some drivers and what not ! : Software Information DDJ-T1 and DDJ-T1: software & firmware updates , Setting File for TRAKTOR PRO 2 download and for other advice then you might have to ask over at the Pioneer DDJ forum.

Thank you for the specifying ! I have edited my post above to mention your post ! Me not having any Traktor and never used any or all of the Traktor software , not to mention not having all your years ? of experience with same, then I of course have a rather limited knowledge on the subject !