Hey Randerson,
Sorry you're experiencing a lot of frustration...hopefully this will become an amusing story before to long. Let me first caveat this with I've no experience with the MPP.
...DID you pay for it by Credit Card??
That's an important question. If you did it gives you a lot of leverage. eBay also has a lot of consumer protections, so if it is a lemon you should have recourse. Helps to know your options before the company tries to snow you into a bad deal such as paying for shipping.
That was my first wish too, I really wanted to see the actual voltages (hate trusting a display on suspect item). Hitting something with a crimper and ratcheting it down tight doesn't mean it's a good connection (e.g., could be a lose wire inside the battery). For that matter, just because the battery display says 54V doesn't mean that's actually what's on the wires. I'd also want to see the voltage on the other side of the fuse. Thought it suspicious it fired up after fingering those.
....You mentioned about seeing 24 volts being displayed on the Mpp LCD.
I don't know anything about the MPP...but did you confirm the model number is what you ordered and that it can handle 48V? A lot of times I'm shipped the wrong model, sometimes good sometimes bad.
So, bad voltage coming in or voltage sensing board makes a lot of sense. Intermittent is odd ... a bad contact? possibly the board or a chip on it came loose in shipping? You didn't try one of my favorite tests for that...it's a solid wallop on the side of the unit to give it a bit a shake. Assuming the incoming voltage was right, I'd have left the battery on and tapped with the back of an
insulated screw driver any bits on the MPP to see if any particular bit did the trick. If you do this, be careful not to short anything and please wear safety glasses... I've had bits of metal shoot off from sparks and caps explode, not worth losing an eye.
.. I sent the seller a link of the video just now. I hope they respond quick. ...
Please keep us informed as to how it goes. If they give you a hard time at least others will know not to buy from them.