I absolutely agree with @acrowcounted - you will see lots of these bags for these prices, but very few actually sell for anywhere near that much. Thats why on some sites you will see these bags linger for quite a while. If you want a better idea of actual resale prices, Ann's Fabulous Finds is a more realistic gage as far as resale websites go (and a lot of bags still linger there, too).
I am not familiar with the site you were mentioning, but I just checked it and they have 1 Birkin 25 and 11 Birkin 30 for sale! I am not trying to prove my point but I do fail to see the relevance of the Ann's Fabulous Find for the topic discussed, which is why are there so many Birkins available on the resale market since they are definatelly not the be found on the above mentioned website!