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I've seen player stalls that didn't change for days in L2. And that shit had TPs and a quite limited range of useful items. I'm more than sure that AoC's markets will move even slower, so checking it every other hour could work just fine.
Either way, people will find a way to make their own kind of AH, be it discord groups, forum threads or player-made sites. And none of that can be bannable, unless Intrepid decide to completely forbid that, which imo would be quite dumb.
Please note that depending on the implementation, what the game does when someone ELSE in the area interacts with a stall and buys something, is broadcast that change to every player in the area so that their client can be up to date without having to recheck the database entry for the player's stall (for example, so that if two players are viewing the same stall and one makes a purchase from that stall, the other's client updates to prevent certain much more complex duping scenarios).
There are reasons why this would be possibly the best implementation depending on multiple factors (some others have lower actual security against duping and bugs, for example).
This information is easily tracked. You might receive it while afk. You might even receive it while standing in your own stall. Encoding does not generally matter because you can teach the sniffer to understand the data packet stream without knowing what the data represents exactly in many cases, since you can crossreference Item transactions to your own database. You can write programs to do that too, they are very short.
You now have enough information, I think, to clarify anything other than 'Packet Sniffing won't be allowed while your computer is running the Ashes client' and 'Intrepid obviously won't implement it that way'. Thank you for assisting as much as you have already.