It's really simple,
http://page.auctions.yahoo.co.jp/jp/auction/148126276 has been relisted for many times so Banjo Kazooie isn't even selling for $500.
Why would anyone, even so called rich people, pay $1,500? That's 300% of a reference price you can't really take as reference.
Two conclusions: a) Banjoo Kazooie is not as valuable (or rare) as sellers may think and collectors are led to believe and b) whoever sells it for 1500 has no intention of making money and is just artificially trying to raise the value (hey, it doesn't work).
Defending this strategy is stupid, why not just support VGM shops with fair prices and a professional business model instead (Amedis, Suruga-ya..)?
I can see why stuff like this
http://vgmdb.net/album/9080 goes for 30,000 yen (and it really does) because there was only ever a handful of copies made but factory pressed CDs, seriously..?