Like I said, I'm not the man to verify signatures. The forgeries are often very good, and the differences will be so subtle, amateurs like you and I would not be able to pick out the differences. For example, the two vertical strokes in the 豊 character in the oshigata you posted extend maybe 1 or 2 millimeters beyond the top of the first horizontal stroke. However, in the sword you posted, these two vertical strokes extend maybe 3 or 4 millimeters above the first horizontal stroke. Is this a significant difference? I have no idea, but it would make me want to find more genuine examples of Yukinaga blade to find out. And what about the 高 kanji? In the oshigata, the 髙 version is used, but in the blade you posted it looks definitely like 高. This difference isn't so subtle... is it significant? Who knows? You would have to look at many more Yukinaga blades to find out. It isn't enough just to say, "looks close, good enough for me".
You have to remember that the guys selling things online are particularly internet savvy, and they are as able to search the internet as well as you are. In fact, its almost unimaginable that they could have an item worth several hundreds, or potentially several thousands of dollars, and are smart enough to post it to an online auction site, yet are not smart enough to search the internet for more info on it.