In practice a lot of fonts (and fonts in anything I have run into here) don't support anything but normal and bold which translates to font-weights 100-500 = normal and 600-900 = bold. So even if it is an eBay oversight, in my browser it wouldn't matter since 200 is the same normal weight as 400 for any font I've worked with)
I couldn't see exactly how good or bad the rendered font looked in your screenshot (even a zoomed in view of the full downloaded image), and can't tell what font that is (nothing indicating it in the CSS you captured and I don't recognize it).
Still not sure exactly what the issue is.
Do you have a URL for a specific eBay page where you think it's a problem?
An explanation of what exactly the problem is? (ie: font is too light and hard to read).
If it's a personal readability issue you could fix it yourself on your end on the client side with about 10 lines in something like Stylish addon in Firefox, directly in usercontent.css in Firefox, or maybe with an addon stylesheet.
Another thing is to consider if you have font setting configured "incorrectly" in your browser (min sizes or choices of fonts non-standard, etc.) If it is only an issue with eBay pages then that would not be the case.
I only work in Firefox, so my solutions are usually Firefox specific (although CSS is universal and the general ideas are the same - only implementation specifics vary)
Sorry not more help at this point.
