I'd buy a lens that looked like this if it gave me pictures that looked like this.

In reality, paint scratching is fine with me, main thing to watch out for are dents and dings, especially on the filter threads (not that the 300/2.8 has them), that's your key that the lens has been dropped or banged, and with faster lenses the alignment of elements is all the more critical.

I've bought a few second-hand lenses off ebay, not all of them were dogs, especially the canon-brand lenses if the seller gives a genuine reason for selling, and/or they have a full kit split up for sale at the same time (like body, lenses, tripods, flash, etc), it may be stolen but not necessarily damaged :P
It's the ones who say "i don't know how to describe it as i'm not a camera guy" and then you see he's been selling lenses for years looking at his feedback, they're the sellers to avoid, you know the lens can't be in good nick then...