Are you buying from a reputable dealer? Then there's absolutely nothing to worry about. Even in the incredibly rare case that the item arrives defective, a reputable dealer will easily help you to get a replacement unit or a refund.
BTW, I buy just about everything online, including major items like TVs and computers, and have been doing so for many years. I've had a few minor problems over the years, but nothing worth giving up the convenience and money saving for. Reputable dealers have always been quick to resolve any problems.