I was the first post in the second post, and yes, transferring junk items via shops is dangerous when done quickly. Generally, TNT is pretty lax to most rule breaking, but there are still flags and obvious maneuvers because they aren't just ignoring rule breakings. They mostly rely on user reports to investigate into things, and Neopians these days like to play detective due to N_T outing posts. Putting a rotting left boot in your shop for 999k isn't natural, and it's the exact type of action that users track and report.
The number one rule of transferring NPs is to take it slow. Constantly moving NPs to your account via 999k shop sells for junk items is moving way too fast. It's better to split up what you want to move and make sure each move is spaced out between a few days. Better yet, you can also try buying items on your other accounts and "selling" that item to your main. Then you can re-sell the item on your main for the NPs.