I'd think most dealers would welcome this type of 
ordering. It would seem to be better than twenty 
e-mails from people asking "can I add this to my 
order" when they saw they missed something. ^_^ I 
know that I have ordered in that fashion from 
Anime-Cel and Anime Chaos, both of whom encourage 
it. I don't even reserve that type of order for 
when I have a strict cel budget - I usually use 
that type of ordering when multiple cels from a 
single sequence are uploaded, since generally, I 
only want one.

There are a couple dealers whose sites don't work 
with this type of ordering, however. Taro's site, 
for example. (But in Taro's case, you would know 
right away if you got the first cel you were 
after - if you didn't, you could move to the next 
one on your own.) Also, Yann's site - but here 
Yann does have a comments section, so I suppose 
you could put the cel you wanted *first* in your 
cart and then list an alternate in the comments 
when you checked out. (That would seem to make 
sense, but I've never tried it.)

The only time I specifically don't order like 
that is from a Japanese site where I know (or 
believe) that the owner is using translation 
software as opposed to being able to read 
English. I'm not sure exactly how I would 
correctly communicate the concept through 
translation software.

Many Sharp Smiles,
--Drac