You're forgetting that magic is not treated just like any other goods. It is highly watched and has essentially "legal prejudice". Also? I don't care how many things saying you're allowed to sell stuff you get, if you sell a criminal a bomb and they use it to commit a crime, you're going to get repercussions. Think about this logically. You would need a license to know how to make Fireball preparations, a license to be awakened, a license to practice alchemy, a license to sell combat spell preparations, and they would need a license to own and use preparations.
You may not be breaking the law, but if they break the law using a bomb you made for them, you will get repercussions for selling a bomb to a criminal. In the least, you will be watched by the law, and no talismonger who works with the shadows any amount wants this. In real life, if a place sells alcohol to a minor with a fake ID, they still get in trouble. I'm sure if you sell a gun to someone who has a fake permit, fake ID, or otherwise does not meet the legal requirements to purchase a gun, you as the seller will be punished as well. Shadowrun would be worse about this, especially for magic, for anyone who isn't working through a corp (such as the suspicious number of Ares products on the street).
That said, it does seem like being behind a mana barrier (which is as simple as being in a magical lodge, and thus easily a rather high Force barrier...) would be enough to stop most attempts to find them. However, I don't see how Guard would do anything, Concealment specifies Perception... Masking is your aura, not your signature, and Extended Masking specifies "preparations in your possession" which also wouldn't include the signature, just the prep itself. Flexible Signature is the one you're thinking of. Flexible Signature and being behind a Ward would probably be enough, yeah.