1.) Use of katakana to write the word "best sellers:"
I think that in this instance, the katakana is being used for emphasis. Best sellers are a very Western conception, invented by book marketers and used to draw a book browser to make a purchase by telling them that "this is what everyone else is reading, so you should too!" I think that in this case, the use of the katakana is Japan's (or more specifically, Kinokuniya's) way of appropriating that same marketing ploy in order to attract Japanese buyers. After all, they could have said "best sellers" using hiragana or kanji or even English, but they chose not to. Instead, its kind of a way of importing Western marketing schemes. (This is all my speculation, however).
2.) Use of katakana in titles: I found two examples of this usage of katakana that I think are quite different.
a.
In this first example, katakana is used to write the title of this children's book. ”バルバル”has no English equivalent, and thus I don't think the katakana here can be considered a loanword. However, "バルバル" does make an interesting sound when read or spoken out loud, which would probably get a Japanese child interested in the book. It is also most likely a name (the name of the fellow on the cover), therefore katakana would be an appropriate way to write it in this case.
b.
In this second example, katakana is being used to write the word "brand" in the title of the magazine Brand Joy. Again, I think the use of katakana here is a deliberate choice because they could have chosen to write the word in hiragana or in Kanji, but they chose to use the loanword form. Looking through the magazine, I noted that it was geared towards selling western merchandise from western companies, so that might be a reason behind this particular usage.
Also, as a final note, why is it that "brand" is in katakana, but "joy" is in English? Something to think about perhaps.