A few random thoughts tossed off at work:

I'll go into a bookstore and buy a book based on 
the cover art and jacket description.  Then, when 
I read the book, sometimes I find I don't like it 
at all.  That's when a lucky friend will get the 
book, or I sell it to a used bookstore near me.

Of course, if I like it, I'll buy the series (if 
there is one), maybe even look to see if the 
author wrote other books outside the series as 
well.

Good cover art (and jacket descriptions) will 
bring me to read books other than my normal 
choices.  I think that cels I buy because I like 
the art inspire me to watch anime I would never 
have selected otherwise.

I buy cels of characters I love.  I also sometimes 
buy a cel just because I like the character 
design, and preliminary research shows that I'll 
probably like the anime, when/if I can find it.  
For instance, the art and manga for Fruits Basket 
is completely adorable, but it hasn't been 
officially released in the United States yet.  I 
believe it's cg, so there aren't cels, but if 
there were you can bet I would have at least one 
of Ayame!  Just from the image and the manga, even 
though I haven't seen the anime yet, because he's 
a cool-looking character (long white/silver hair, 
just like Yue!)

However, I don't think the 'dog in the manger' 
attitude, buying a cel because one knows that 
someone else, who is an avid fan of the series or 
character, will spend more money to get the cel, 
is right at all.  That is just an unkind way to do 
things in a hobby.  There really does seem to be 
an overall cel karma at work sometimes, doesn't 
there?  It's better to be nice!