I've bought several spare LiIon batteries for my MiniDisc recorder from Hong
Kong / China at 99p each plus around £8 postage.  (Total cost around 1/3 UK
shop price.) I think the 99p pricing saves the vendor on fees paid to ebay.
Haven't found any difference between the privately imported ones, and that
which came with the recorder, and I presume is genuine.  (Impressive looking
postage stamps.)

You may get additional cost if item stopped at customs but that may be
happen.  I think you are unlicky if they pick it out, like when I
ordered from Hong Kong!

If you are buying from a trusted seller with at least a 99% positive
feedback rating, you can be pretty sure it is genuine.  I have bought
some big ticket items from Hong Kong and never had a problem.

I've never heard of Marumi, but I am a regular customer of Cameraworld
and rate them very highly.  I don't think they would ever sell junk,
but I repeat, I have never heard of the brand.  Some expansive claims
were made when Kood filters were launched on the UK market, but they
were absolute junk, so beware.

My own recent experience is limited to Nikon, B+W and Heliopan
filters.  Several years ago I dabbled with Hoya HMC, Sigma and Tiffen
and I would never touch any of them again.  Nikon filters are reported
to be made by Hoya but the Hoya HMCs are almost impossible to keep
clean while the Nikon filters are a breeze.

In circular polarisers I only use the B+W Kaesemann type.  They are
epoxy sealed at the edges and last a lifetime if taken care of.  All
other polarisers deteriorate from the edges inwards because they are
not sealed, and eventually moisture always enters the sandwich (they
are made of two glasses separated by a plastic mesh).  If you use
cleaning fluids the deterioration can be very rapid indeed!

Unfortunately, they Kaesemann type are very expensive - about 50% more
than the already expensive B+W standard polarisers - but you only ever
need to buy one, because it will last forever.  You can get them at
surprisingly competitive prices from Amazon.com and B&H PhotoVideo in
the USA.  

Go to
http://www.amazon.com/
or
http://www.bhphotovideo.com/

and search for "kaesemann polarizer".

A B+W 77mm costs $152.95 at Amazon, which is about £77.  B+H charge
$164.95.  Remember that these are the best polarisers (polarizers)
that money can buy.

I've had very little problem with HK stuff; they sometimes price low and
make their margin on the postage. I had to send something small back
recently and was astonished to find the airmail was only about £1-20.