Hi 31fortwo,
Here's a thought! Dunno really..cuz it's just a wild-a_s-guess!
But Auctiva says they "resize" photos to 400x300px for listings. Now your camera and photo-equipment sounds way superior to my old camera stuff..so maybe your photos are really way higher in resolution than auctiva expects to have to "resize" (meaning that their algorithms could be messing with the resized-renderings. (?)
The clue here is that you say when you supersize then the photos are fine. Ok! Well auctiva apparently doesn't 'mess' with the supersize photos (in other words, doesn't 'resize' them). Therefore you can deduce the original photos arrived, and are ok.. But because they are very high-density, the computer-graphics algorithms which are evoked for 'resizing' may be somehow choking on your photos for that very reason.
And BTW.. what are you using to 'upload' with? Aren't there options for the transport-tool used to upload. I thought there were..but I only ever used the simplest choice which is plain old html transfer (or whatever it is).
suggestion: just try to upload and test a couple shots taken at significantly lower quality (or size). Maybe you'll get a better clue from that!
I dunno..like I said. I'm not an expert here..my input is only really just a WAG.
cheers!