Its obviously something with eBay's pages that is creating the issue. I just confirmed I get the same odd, jerky text shrinking. Although its kind of a separate issue, but related, I find it a little strange that mobile safari is missing what is in every browser I've ever used: in other browsers, there are always preferences that allow force-adjusting the smallest point size that the browser will display, even the desktop full version of safari one can adjust this.
I have been using a workaround for the lack of a "Smallest point size to display" in mobile safari (or, depending on how you look at it, the inability to adjust point size in any way).
http://tinyurl.com/c3gsu58
I have set up several of these bookmarklets, and it seems to work well. But as my eyes are getting worse, it is becoming tedious that for every website I visit I have to experiment with my bookmarklets to determine the ideal point size for comfortable reading, and I also doubt it fixes the eBay issue of text spontaneously shrinking, but I thought I'd post the link anyway... you can google around for others explanations of the javascript bookmarklets, there are many available.