It is never OK. If brick and mortar stores go out of business there will be no place to demo equipment. Then you are just buying blind. I won't buy a piece without hearing it in my system. Everyone needs to support their local dealers if they can.
I am not rich and buy almost everything used. I am friends with a couple local dealers. I usually buy stuff that was traded in by other customers. This way I can demo something at home before I buy it. The dealer will also stand behind the piece if there is a problem. My ARC dealer is great, he will call me if he is getting in a piece he thinks I might want. I usually tell him my plans if I am looking for something in particular. One of my local dealer let me buy a pair of his demo speakers at a great price and make payments. Even though I am not buying new, they are still making money off of me and I am supporting their business.