I don't have any firsthand information about that situation, but as far as I've read (before I decided against the 645z) Pentax is very popular in Japan hence the increased supply.
Apparently most Japanese camera stores are trustworthy, but my one experience with a Japanese camera store (with a 99.5% positive rating, relatively fine) buying a Rollei lens was a negative one. The price was on the low side of reasonable but the lens was described as being 100% functional. What the seller did not note was that there was oil all over the internals, and the shutter blades were damaged. I complained and got a small partial refund, which did not even cover all of the repair cost.
Anyways, long story short, I'm happily using the lens (when I do) but definitely buyer beware - they charge significant shipping (which is reasonable but which you will never get back) and returns are often uneconomical, so if issues come up one is sometimes SOL.
I'm not sure the savings are significant enough unless it is a hard to find lens, but if you find a trustworthy seller let me know. There's a lot of glass in Japanese stores that does look great, for many cameras.