Unless I am mistaken that used milltek Add has been running since Xmas 2014 or at least Spring 2015!
I know that because I was looking at that as an option back then just for the Anni system & flog on the downpipe.
IMO The price is about £100 too high, the K04 downpipe is desirable, but under £300 is a more realistic price, especially as its collection or own courier.
You could contact the seller, point out that their listing has been running for a very long time and make an offer either for the whole lot or ask if they could split the system from the down pipe.
If you end up with the lot, just re-sell the downpipe/spots cat on its own.