Interesting question!  I've been on both sides of 
the fence...

Example and Consequences of Perspective One

Before I ever watched Utena, I adored the cels in 
other people's galleries.  But I never bought any 
that were up for sale because I hadn't watched 
the anime.  I was very rigid about that rule for 
myself.  After watching the show, I fell in love 
with it and have since started collecting the 
cels...and am kicking myself for missing out on 
the ones that I did.  I don't know why I ever 
swore off shows I hadn't seen.  It was a big 
mistake.  

Example and Consequences of Perspective Two

A long time ago, Drac posted a cel of Muse from 
TnN.  I knew nothing about the series, as that 
was the first time I had heard of it, but I loved 
her cel and character design!  So when Yann 
updated next with a prime A-level shot, I jumped 
on it.  I had never watched any episodes (I've 
only even seen three by now), but to this day 
that cel remains a favorite in my collection. 

Anyway, now I know better than to forgo cels from 
shows I haven't seen.  I naturally focus more on 
the series I have watched and like, but that's 
more of a financial issue than moral one.  Bottom 
line: buy what you like!  Then you won't end up 
with regrets, like...

Why didn't I buy Lady Zatara's face-shot Saionji 
cel?  WHY?  *cries*