Heya,
I shop many of these often and just generally price watch.
The 200 F2.8L II generally goes for $500ish in really great condition, $450 with some wear. This lens is great, but it's not as popular as the 135L and the now ultimately 70-200 MKII zoom. So this lens largely just sort of sits the side lines so to speak, and doesn't command a higher value. The 100L & 135L fetch higher used prices. And even the new price of the 200 F2.8L has dropped lower than that of the 135L and 100L funny enough. But their zooms did largely take a big chunk of interest out of this lens as the MKII is as sharp, if not better in many ways.
The 300 F4L IS is still relatively new. It generally can fetch a $650 price tag. It's less popular though compared to the 400 F5.6 and the 100-400 MKI which compete with it's price tier. The non-IS version still fetches around $400~500 in good condition. The IS version gets a bit more. Again, this is largely just a product of being a less popular lens in the white lens game. Great lens though. I actually prefer the 300 F4 over the 400 F5.6 for birds in general. It's a good daylight sports lens.
As you pointed out, there's what people list them at, and there's what they sell at. Very different numbers.
Hope that helps. 
Very best,