$225 sounds like a pretty decent deal but not amazing? If it's lower than other prices you are finding used, then go for it.
On a side note, based on the product description (sorry, I haven't used it myself), I'm not sure if the Audio-gd FUN is what you're looking for if you're aiming for a neutral, more accurate presentation overall. An amp with no feedback is kind of a bold design, to say the least. According to them, "Most people had know the global feedback can offer better specs for the meters, non-feedback can't please the meters but can offer better sound for human's ears ." I'll grant that "better sound" is subjective and such a design may be appreciated by many, but if you're intentionally not pleasing "the meters" then that means you're intentionally adding distortions in the signal chain. That objective maybe clashes with wanting a reference can for listening?
Then again, I'm sure lots of people like wonky DACs/amps with more neutral headphones, or more colored headphones with more neutral sources. Just saying.