At heart, the OP has asked a simple question: are power cords assumed in the purchase of used equipment? For the most part, the responses here don't address this question. After all, the vast majority of electronics DO include a power cord as part of the original sale. With that in mind, wouldn't it make sense for a seller to indicate any change from the original, new product?
I agree that it's not worth getting worked up over a missing power cable, and I can't imagine sending back a used amp over it. But I'm interested in the question:
Is it reasonable to assume that a used piece of equipment will include a power cord?
(I get it that some people would consider this as part of their research, etc. But that's not the question here.)