A few of my photos on IStock have received ratings from other contributors.  In each case they gave me a 5, which is great of course, but is there any real significance to this, say, in terms of search rank? Or is some other game being played, about which I'm clueless?

This is something frequently discussed in the forums - my understanding is that  ratings did play a part in the best match at one time, but some contributors collaborated to form "ratings gangs" to help their positioning, and the current word is that there is no longer any effect - unless of course you change the search order to "By Rating".

In theory, your rating just means someone likes your image - but it's often been mooted that it would be better if there were a simple "Thumbs Up" rather than a 1 - 5 rating, as it's generally felt to be bad form to rate less than 5, and a mortal insult to rate as a 1!

Yes, that's quite true.

My top seller has no ratings at all - those with many high ratings have sold many fewer.

Not necessarily, most of my rated 5 images are nither landscape or scenic. The rating does not makes them all best sellers though.