One little feature of a megadungeon - and some video games - that I've never used in play is the "dungeon shop."
I mean a store, inside of a dungeon, usually selling things immediately useful in a dungeon (rations, 10' poles, torches) or special equipment you can't get elsewhere (magic armor and weapons, special magic items, useful monster bits), or buy the same. In video games they tend to have hideous guardians that prevent you from reasonably robbing the place.
This is one of those things I've heard of in tabletop games but never encountered.
Have you ever used one in a tabletop game?
My DF games feature a lot of in-town sales of equipment one might consider "special." An in-dungeon store wouldn't serve any purpose except to replenish stocks of forgotten items. The PCs would eventually assault the place anyway, and kill everyone and take everything, unless I made that literally impossible.
But I'm curious about actual real-world non-video-game occurances of mid-dungeon shops.