You are correct, many people do sell on full bags. Many people also sell after raids, dungeon runs, and quest lines. I feel the better assessment is the sell on full/auto-port theory may be setting the stage for further automated inquiry by Trion. Or the Dev's left in some hacking code to teleport...
Think about it, you and the millions of other players all sell to vendors. How many of those millions of players wait until their bags are full to sell? How many have been online for more than 10 hrs and wait until they have full bags to sell? Now you have the metric down to a significantly smaller amount. How many have not completed a quest, typed a single word, or grouped in the last .... I think you get the picture of how it could be a metric to flag accounts. While I agree my theory is a bit left field, it's one thing most bots have in common.