When you look at:

http://db.anime-
taro.com/db/taro/table=produce&template=new.html

It tells you what has been updated in the past 
24 hours AND how many cels are still available 
for each title.  So when someone puts a cel into 
their cart, the quantity automatically is 
updated (and Keep-Track does register these 
changes as well as updates).

So it's just a simple matter of monitoring the 
link above, I think.