CNC is a hobby for me and designing stepper drives also.
I have a 30 years career in electronics repair and know where the weak points are, where they save $$'s using cheap components resulting in premature failures.
Good electrolytic capacitors and trimmers are expensive, my employees and me replaced several thousands of these, no kidding.
We had to give warranty after repair and for some models of computer terminals we simply changed approx 20 components to be sure they wouldn't come back.
Big problem is that a simple electrolytic capacitor fails and sometimes causes 10+ others components to blow at the same time, avalanche effect and thus an expensive repair due to a 2$ cent saving.
Back to the topic:
I have designed several THB6064 drives for me and MassMind.org, a TB6600 drive is coming soon.
For questions like the one below it's best to know exactly what the possible problems are with the cheap Ebay drives.
This is the reason I bought one to analyse and compare.
PS: I'm now working on the schematic of this drive and it looks like I bought a heatsink, some connectors and 2 optocouplers in a socket.