The power button doesn't work while off but it works while on.

I bought this laptop a few months ago to refurbish it (SSD & ram upgrade). The seller looked very reputable, with a high average rating on eBay and detailed pictures. The ram and ssd upgrades went fine, and the computer ran perfectly for a few months, until it stopped turning on. I though it might have been a battery issue but was confused because it only had about 13 cycles on it or something low like that. I disconnected the battery and plugged it in with the magsafe. It turned on as soon as I plugged it in and it worked fine. I then assumed it was a battery issue and ordered a replacement from ifixit. After installing the replacement to no avail, I began to suspect it was a power button issue because the only way it would turn on was if I left it charging for hours, it might turn on randomly (maybe old house electric outlets?). I arranged for another battery to be sent to me to rule out that as the source of error. It didn't do anything either. So now I think it must be that the keyboard is corroded, but when the computer is running, on the desktop say, the power button works fine. It puts the display to sleep and opens the shutdown menu without fault. I am examining the logic board for signs of liquid damage now, and I have found small areas of what looks like rust on parts, residue around some components on the board, and on certain screws are areas of blue buildup. My warranty is long expired, but do you think it's worth it to pursue the company or should I just replace the top case? Thank you in advance and sorry for the poor image quality