On that second trade, it looks like Army of Snails didn't buy it from that seller, he went and bought it from someone who had their own trade up for 5 keys if you look at the Outpost item history.

Of course, we have no idea what it really sold for, be it 13 ref, 5 keys, etc. He was willing to pay 13 ref, at one time I suppose.

This is such a low volume item that you could literally track down every open trade right now and post it as evidence without too much more effort. I had a quick look and saw a few that looked like they sold for 6 keys, so to be fair I think you should consider those as well.

edit: Also, here's one up for about a day at 4 keys, let's see if it sells quick!