I have bought and traded thousands of used gears from China, the right source will always want to keep their name clean and thus they would still offer you DOA, when I first started dealing with my main supplier in China, I had to record video of me unpacking and testing the gears, and they also sent me videos of their own testing in China, overtime some solid trust was built and it became a matter of me telling the seller than x hash boards or PSUs were bad and they would send a replacement without a second question.

For obvious reasons, the main supplier I used to deal with always sold slightly more expensive gears, you find S9s selling for $90, yet they sell them for $100 or even $110, that extra money was reasonable because they did proper testing and cleaning, so the failure rate was pretty low like 1-2 hashboards in 100 batch miners, I did take some risks of buying from less reliable sources and ended up with gears in terrible conditions.

Overall, I'd advise against buying used gear especially from overseas.

OP, still waiting for your proof about them promising to pay you something.