Back in the days before I semi-retired we used to sell computers. For a retail home machine, the gross margin was twelve to fifteen percent, and from that you had pay your overheads and staff costs, make a profit and cover the cost of the free technical support that everyone seemed to expect (a bit like buying a car and expecting free driving lessons). On the other hand the electrical chains would sell TVs at a gross margin of 30 to 55 per cent, and all they had to do was deliver the box and forget it.