That doesn't have anything to do with the text encoding; even if it were selecting the wrong one, basic ASCII will render correctly in just about any character set. Further, eBay specifies fonts for everything, so your default font isn't going to have any effect on that, either--that only comes into play if the site doesn't specify a font.
From the looks of it, that's just some oddball font with upside-down characters--whether it was intended to look like cyrillic or it's just a silly font, I don't know, but it must be one you've installed because it's not system standard to my knowledge.
And really, that sounds a whole lot more like a system-level font problem than something Safari 5 is screwing up with stylesheets--since all the text being mis-fonted is Arial plain, it's quite possible that font is in some way being misidentified as Arial. It has no bold version, and since Safari refuses to create pesudo-bold/italic fonts for display, the text goes normal when it's bolded.
This exact behavior is what usually happens when you've either installed a font, or a font has gotten corrupted such that it has the same font identification as a system font. For example, if you install something that mis-identifies itself as Lucida Grande, your menu bar may show up in that font.
Two things you can do:
1) Open up Font Book in your applications folder, select everything, and do a "Validate Fonts" from the file menu; see if one of the ones that comes up with an error is the problem font; it may be reparable.
2) Figure out what specific font that is, and locate it in Font Book. I'm going to bet it shows up as being a duplicate in Font Book, but even if not, try disabling it. Your problem should immediately clear up, unless it really is something else going on. You may need to quit and re-open Safari for the font changes to take effect.
You could also try using a system utility like Leopard Cache Cleaner to delete font caches, but that probably won't do anything unless you also disable the problematic font.
