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).
In addition, as some of my helpful commentators have pointed out, using katakana to write "best sellers" here can also be considered emphasis, almost as if they had put the phrase in italics. Another aspect to be pointed out is the presence of the English written underneath: why use both katakana as well as English to display the same things on this sign? One of the comments, Matt's I believe, suggested that this might have been used to draw in English-speakers or bilingual speakers as well so as to make it obvious to them that the books displayed might be currently popular reads in Japan. This brings up another interesting question about this display: is "best sellers" here pointing to Japanese best sellers (as in best selling books currently in Japan?)? Or American bestsellers written by Japanese authors? Something to ponder perhaps (sorry, this edit resulted in more questions than answers I'm afraid).
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.
One of my comments points out also that while "バルバル-san" here is most likely the little boy's name, the sound aspect of the word would surely be a draw from children. In addition, she points out that katakana is one of the first alphabets that young children in Japan learn, thereby confirming that the usage of katakana here is meant to appeal to young readers on many different levels. Finally, she brings up some other examples of the usage of katakana as onomatopoeia that goes beyond "バルバル-san," such as the usage of katakana to express the sound of the butterfly's wings flapping.
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? It is quite possible that this mixing of English and katakana (as seen in the first example, with "best sellers") is another deliberate marketing ploy, geared towards expressing the international aspect of the magazine as well as the magazine's merchandise (according to virtualjapan.com, Brand Joy is a magazine specializing in the display and sale of Japan's most popular luxury merchandise brands, many of which are Western companies such as Gucci, Prada, Louis Vuitton, Dior, etc....). In addition, one of the comments brings up a really interesting and important point that I did not originally consider, that of the usage of well-known English phrases that would appeal to Japanese audiences. According to the comment, "joy" is an English word that many Japanese people know, and thus it's appearance in the title of this magazine wouldn't necessarily keep Japanese buyers away, and would in fact be a possible draw.
Sources:
1.) Kinokuniya Bookstore, USA: http://bookweb.kinokuniya.co.jp/indexohb.cgi?AREA=03
2.) Virtual Japan: http://www.virtualjapan.com/wiki/Brand_Joy_Magazine
3.) Brand Joy Magazine: http://brandjoy.sv1.siteclutch.com/
