Buying used vs new?

If it’s a POS, then half off is not a good deal.

if it's something without functionality you want, then half off is not a good deal. 

Because it's impossible to differentiate between used and abused.

For reputable equipment always buy used if available in good condition. Saves you the 6+ months running in. Dealer amps and speakers likely current models and unlikely to be abused. Not like cars. 

I have bought used and new.  I usually contact the manufacture directly on new purchases (smaller north American companies) to get a better deal.  One absolute is service.  It doesn't matter how good a deal you got if it can't be serviced hopefully by the manufacture in north America.

For me, just know the source.
I am less trusting of trade ins unless single owner.  I would rather buy from a trusted seller personally.  Or local.  Local is best.