I have never bought off the website, but I did 
buy a large quantity of doujins from the actual 
store located in Osaka. They don't bother rating 
them there, and they come sealed in plastic, so 
you have to decide to buy it basesd on the cover 
alone. 

Needless to say I accidently ended up with a lot 
which were too raunchy for my taste. However, I 
turned around and sold the ones I didn't like on 
ebay. I found that the more explict the pictures 
you include, the more likely the item is to 
sell. So it would be in the store's best intrest 
to use the most explicit scene featured when 
showing samples from the book. 

So I'd expect it's pretty safe to go by what is 
shown in the sample, and it's unlikely there 
will be worse content than what is shown. (and 
as I discovered, if you don't like it, the nc-17 
doujins are *very* profitable on ebay, so it 
should be easy to get most of your money back 
that way if you don't like it)