If you buy the parts alone and assemble yourself, you'd probably only save around 100 bucks or less, and you get warranty and peace of mind from them. Thing is.... new GTX 1070 Ti is selling for at least 700 bucks, but for that price, or topping up 50~80, you can get a GTX 1080 Ti which still has a few years warranty left. Just check around this forum or Carousell, there are loads of them, from the recent mining crash.
Maybe you could negotiate with this PC builder and swap out the 1070 Ti with a used 1080 Ti that you buy yourself.