Have you tried to check this site
https://www.asicminervalue.com/You can see all profitable ASIC machines if you are not paying on electricity bill then this is a good choice and Im sure you will make more profit than buying a 2nd s9.
This is just a suggestion because there's no guarantee that the 2nd hand s9 will live long in mining.
I use that site but problem is where I can buy...
Official sites are out-of-stock, vendors there are completely scam. I have bought on cryptouniverse.at this Sunday and still Processing, I dunno if it's scam or out-of-business.. anyway I will see.