I have noticed that to some extent, yes.  I think 
the reason behind this is not very mysterious 
though.  For example, when the SS4 Gogeta was put 
up on Yahoo Japan there were also about 4 or 5 
more SS4 Gogeta cels  that followed on YJ in the 
next week or two.  They were all being sold by 
the same person though.  Also, DBZ cel prices are 
high, and if a collector sees that someone else 
sold their Gogeta for a huge profit...he might 
consider selling as well.  Also, about the SS4 
Gogeta cels--there were also suddenly a lot more 
out there (galleries, stores) because Anime Taro 
updated with 5 of them in one night.

The Vegita, Nappa, and Radittz thing is 
interesting though...I also had never seen a cel 
of all three of them together.

One thing I have noticed a lot is that DBZ cels 
tend to be released in bursts.  Example--last 
week there were about 45 DBZ cels on YJ.  Now 
there are over 100!  The main reason behind all 
of these questions is probably that sellers will 
try to sell a lot of their cels all at once.

Just my opinion ;)

-Graham