Yep, USB cables are standard in connector's shape but not all are equivalent in the connection. Some uses better cobbler than others, shielding may vary too and, as @Hartmut_DK5LH said, some cables have only 2 out of 4 internal cable (the + and -, not the data line). Some are charging cables, others are data cables …
It goes the same with power source unit for raspberry pi. Some are better than others. I've tried using official power source for raspberry pi but it did not work so what I did at the end is, as you said, soldering a small circuitry (namely, a LM2596) to power the pi from my printer (note: I'm a total rookie in that, and still it was very easy). It was not about modifying the printer, actually – I didn't have to unscrew a single screw of the printer, I used a Y-shaped XT60 cable, if you wanna look on the net ;). "Chargers" sold on Amazon are generally cheaper than real power source units (which have to be precise in their current delivering; a USB charger does not really have too, I learnt).
But first things first, I'd probably try to get the connection to a computer working. I'd probably try different USB cable on a standard computer until I get a serial line to the printer, to rule out the raspberry pi.