Unique iPhone Power Problem

Hi all! I have an interesting problem with my iPhone 6. Let me give you a little backstory first:

Bought a used iPhone off of Facebook Marketplace (first mistake), from a guy who said that it just needed to be restored in iTunes. Seemed like a simple fix, $80, why not? Got home, plugged it into my PC, and nothing happened. Literally nothing happened. I put the phone into DFU mode, the PC made the little "inserted" noise, (The screen of the iPhone never came on) iTunes came up and said an iPhone in recovery mode has been detected, I clicked restore, then later on I come back and it had given me an error code. (4005, I think.). Thinking it maybe needed to charge, I plugged it into a wall outlet. The apple screen came right on, and it went to the iTunes recovery screen. I let it sit for an hour or so and tried it again. Same problem with iTunes. I called a buddy of mine who fixes stuff, and he suggested it was the battery. When he opened up the phone he found a considerable amount of water residue. We tried a new battery, and that did not fix any of the problems. So, I contacted the original owner and asked him if it had ever had come into contact with water. He said it was in a sauna once, but nothing else. I checked the water damage tab under the sim tray, and sure enough, it was red. So I looked up how to fix water damage, gave each individual part a 91% isopropyl alcohol bath with a good scrubbing, re-assembled, and still had the same problems. So, since a lightning port assembly is so cheap, I replaced it, and still having the same problems.

Other bits of information:

• Yes, I have tried all the USB ports on my computer, and tried this process on a different computer. No difference.
• One USB port on my PC will actually bring the screen on during the iTunes recovery process, but then endlessly bootloop.
• When not plugged into that specific USB port, every once in a while (read, "rarely, as in, two times ever"), the screen will finally come on when plugged into the computer and show the iTunes recovery page. When iTunes recognizes there is a phone in recovery mode, it starts the process and then the screen on the phone goes black, and eventually gives error 4013.

In Summary:

• Bought a water damaged iphone
• iPhone turns on and goes to iTunes recovery page only when plugged into a wall
• Computer only recognizes iPhone when in DFU mode, but the screen does not come on, and I get stuck on "waiting for iPhone" in iTunes until it errors out with 4005 and sometimes 4013.
• Replaced battery and lightning port assembly, nothing changed.

I'm at a loss. Any ideas what I can do?