Hmm ... they really sell out the stadium without giving a chance to purchase them online? Clubs(concerts, events in general) usually have qvotas of how much they sell in which system. First the fans but the tickets are usually limited.
Granted if this was the case you would probably have to be online the very moment they go on sale but still ...
Well it will only be a small allocation, as it is an away game, so whether the stadium is sold doesn't really matter (unless you want to buy tickets and watch with the Romans).
For the low-interest home games, I think the system you described is similar to one they use, because it is always the same areas of the ground that are for sale. Big home games are naturally different, and get given to ST and Cuore Rossonero holders.
But an away game is a few thousand at most, and they will be snapped up by the hardcore ST holders. Also, I'm not sure they sell away games online to fans outside of Italy, because of the tightened security (indeed, this will be impossible with Tessera del Tifoso).