I wouldn't worry about it being a fake, the cameras that fakes are made to look like are Leicas. I would worry about it being in the condition that the seller claims it to be. I'm not saying that your seller is dishonest, but it's too easy for people to misrepresent (OK, LIE about) the condition of a piece of equipment, especially from 10,000 miles away, and especially if they know you are 15 yrs of age, sorry to say.
Get the description of the condition from the seller in detail, and get pictures. Someone that may be dishonest will usually not want to bother to give a buyer extra emails describing condition, nor will they send detailed pictures unless it's in good condition. If someone can't be bothered to maintain a dialogue with you, then forget about buying anything from them. Also get a return guarantee, altho that will be expensive for you to mail back to Japan.
Unless you can be sure its a good deal for you, limit your bidding or wait and buy from a seller here or an actual store in the U.K. Like just about anything else, they do come up here regularly, just be patient.