I did not formulate a problem - so it is understandable that I can't rate any of the answers as a solution. What I did formulate was a warning: be aware that when one buys Apple stuff on eBay, not all property rights transfer to you. Some are retained by Apple. There is no way to avoid this as all the respondents appeared to agree. I have had to buy another computer (although now a warned man!) and abandon the failed one. I can scavenge a number of the latter's parts.

I very much appreciated the efforts to respond.