I've bought a concert stereo from ebay for my tt ,but old owner doesn't have security code details

Hello Stephen,

There used to be a unique number on these sets, and armed with that - again used to be - there were a number of on-line facilities that provided you with the correct code for a fee. This service may also be available via. Audi dealers, but I wouldn’t be surprised if the original VIN would be a necessity - so back to your EBay seller? 
Let us know how you get on Stephen.

Kind regards,

Gareth. 

Use the mobile app 'Radio Code Generator for Cars.' Get your radio out of the housing to get the serial number off the sticker on it. Input your serial number which will start with AUZ........ and email address they will send a code usually within a few minutes. Mine took 5 minutes to arrive and it worked no problems. I think it cost around £6 no vin necessary.

Fred