For one thing, first impressions count a lot.  
What I mean is that cel collectors (and fans in 
general) become fans of the originating series, 
whether it is a TV series or an OVA.  For Fushigi 
Yuugi, the TV series is what most people fell in 
love in with first and thus cels from that hold 
the most value for them.  On the other hand, the 
Lodoss OVA is what sparked the popularity of that 
title, and therefore cels from that is much more 
highly sought.

Sequels or spin-off series are therefore less 
highly sought, in general.

Second, a big consideration is availability, or 
to put it more succintly: how rare are the cels?  
OVA cels tend to be shorter and thus present 
fewer available cels than their TV counter-
parts.  This is one reason why Rurouni Kenshin 
OVA cels are just as valuable as Rurouni Kenshin 
TV cels.

Age is also a major factor to the second issue.  
The older a series is, the more rarer and 
valuable the cels tend to be.  For instance, 
Fushigi Yuugi TV and Lodoss OVA cels are older 
and much more rarer than their subsequent counter-
parts.

Third, the quality of the animation could be one 
of the deciding factors.  In general, a movie or 
OVA series enjoy a bigger budget per 
episode/project than a TV series, and therefore 
the level of quality tends to be higher.  Thus, 
cels from a movie or OVA series could be 
considered more valuable.  One of the exceptions 
to this trend is Fushigi Yuugi, where the TV 
animation is equal or often times even better 
compared to the OVA series.

One thing cel collectors always take into 
account, though, is the quality of the individual 
cel itself and also it's importance (scene-wise 
or character-wise) to the anime.  Most of the 
time, this will overshadow the three 
considerations I mentioned above.