Keeping silver polished is a big pain compared to a gold surface, with no indication of the thickness or weight of the plating you don't really know how fast it will wear off (assuming it's 24K as advertised)

I have the impression that this 7 year old Japanese site is some sort of Ebay like marketplace for individual sellers, which makes it more risky than a straight store purchase.

It isn't "fake" it is gold over silver. Very common (called vermeil). I actually have seen some great pieces - and, of course not being solid gold, much less expensive while still 100% precious metals.