Thats a decent price,  if it's a trusted seller. Only downside is, how long will a used S9 last, before there are issues? Most used S9s are in this price range.

Highly recommend going with a Canaan 841, for a little more money, you get a 2yr warranty and much better reliability. Take a peek at
the specs. Definitely in S9 territory.

https://blokforge.com/product/avalon-841/?utm_source=BlokForge&utm_campaign=7a55741c4b-EMAIL_CAMPAIGN_2018_03_14&utm_medium=email&utm_term=0_6acd6e47ee-7a55741c4b-68080073&goal=0_6acd6e47ee-7a55741c4b-68080073&mc_cid=7a55741c4b&mc_eid=b640a88382