You may be using the wrong stall. If you talk to any of them then you will just get a white screen if you are buying something from them. Yours should be the one that you see has a chest next to it on the ground. If that’s the one you’re at let me know and I’ll take a look. You should just see a menu there right when you walk up, no need to wave at them.
So the bosses no longer have any specific loot. However, they have a much higher chance to drop Rare-level version of the loot at this level.
Once you get to Level 20 content, then the bosses have a much higher chance of dropping Epic-level loot (which regular monsters don’t drop).