So, I found this neato circular shop room. I can take this Scaled Armour piece without Brax blocking me from anything, presumably because there's supposed to be a blocker here. This triggers his block-the-exit reaction, which obviously does nothing since I'm outside the shop area. What will happen if I try and take it with me? This is one of my best characters so far and I'm on permadeath or I'd just try it... and indeed I might before I leave the floor, but is there a known reaction?
Leaving the shop area by somehow bypassing Brax, such as teleporting past him, pisses him off and summons the collectors. That layout of the shop just makes it easier to slip "past" him.
Look on the minimap. According to said map, you're still standing inside the shop's radius. You can pick up the item and wander around, but once you step past that pink zone, you're screwed.
Re Brax, how far does his "steal check" extend? The whole level or just x squares outside his shop? I did notice that if you use Move in Mysterious Ways only, you can escape a level without triggering his steal check. Teleport, spam space bar, teleport, sweet larceny goodness. Does that work for all teleports as well?
I believe the rules are "if you do anything that changes the music to stop being the shop music, you'll make him angry". This is basically moving (by virtue of WASD; teleports don't count) into a tile that is not pink on the minimap. Or using a spellmine near him.
But only sometimes. The Blinding Flash proc from Vegan, as well as several tome procs, don't provoke him at all. At least, they didn't on my last run.
Incidentally, you CAN place a mine/trap on the tile he WILL teleport to (in order to block you) prior to picking up an item; upon doing he so, he will move to block, trigger the trap, and become angry =)