Regarding the board, there's 2 spots for the pcie connectors. Only one of them is populated, the bottom-right. Top right may have been populated, but it's not likely--there's merely some solder on 1 of the traces, with 2 of them having just a smidge. The 3 areas with burn/fingerprint marks appear to be test points, or otherwise not primary power points.
If they're fingerprints, it's a very high coincidence for them to be sitting right over those pads. Instead it's possible that the reflow oven they used didn't put any solder there, and they used an iron at far too high of a temp to do so by hand. Or it's smudges, or it's something else, but those traces don't appear damaged so there'd be no point for the PCB to be burned there.
Further support for the high-temp hand-hack job is in the bottom-right pcie connector, look at the burnt flux. A clean, proper-temperature flow will not exhibit that.
Very doubtful that the board has been used much.
It looks like only 1 PCI-E power connector has been installed. The hand soldering is expected, because it is a through hole connector placed on the back of the board - a non-standard configuration. Normally, this type of board will be IR reflowed on the top and wave soldered on the bottom. This can't be done with a back-side connector in place, so the connector will have to be hand-soldered as the last step in assembly.
The discoloured areas are signal connectors - the PCB CAD files show that they are miles away from anything power related. Those are probably the coldest spots on the board, being well away from power circuits and from the ASICs. I cannot explain the discolouration.
The board is unlikely to be operable (at least at meaningful hash rate) without a large heatsink - there is no evidence of scratching on the heatsink mounting holes, which would be expected if a heatisnk had been installed and removed. There are no traces of heatsink compound left - so if the board had been used, it has been thoroughly solvent cleaned before photographing. Prolonged use at high temperatures also degrades heatsink compound, with a tendency to make it release silicone oil. Anyone who has serviced or rebuilt graphics cards in mining duty would have seen this. This silicone is very difficult to clean off, even with electronics solvents like IPA.