I got around my 500 error ... my fault - I had reverted back to the original files using an older version of one of the php files ...
BUT then I decided to re-install, thinking that I had made the manual installation changes to the wrong version as well, so I made sure I had the right version this time
and tried again.
Alas - no go. The installation failed on the same 3 files, and following the instruction to manually make the changes did nothing to change my first result ... as soon as I clicked on the ebay settings option, the system came back with an error. But then I found ONE of the problems. The installation instructions for the event that you have to manually change the files omitted a "comma"! I fixed that, tried to run the ebay settings again, but alas, now I get that 500 error again.
OK - enough is enough. I backed out of the install (successfully this time!) and will try it again in a few months when things are hopefully more stable.