We post defaulting details in the public forum all of a sudden that cheapo alarm system you have becomes worthless, because any burglars reading this will know how to default it.
Also, ADT have different engineer codes for different areas, even if we could give you an installer code its likely it wont work, you purchased it on ebay, the chances are it wont have come out of a site in the same town the vendor sold it from. You need the postcode of the house it was originally installed in and an engineer that works for that particular branch, chances of that happening is about 500/1