For most of the game, my rundown old store actually never appears to do much of anything. You find a few books lying around that you can browse through, sometimes adding entries into your adventure journal and offering hints about a variety of side-quests. Each time you speak to me directly, I usually just tell you to leave me alone, or make a snarky insult, though with persistence I start offering cryptic dialogue or make different sarcastic comments (possibly breaking the fourth wall, depending on the type of game) about the current events of the plot and the lore of the world. You might even encounter unusual monsters if you remain on the premises for an extended period of time.

Once you start reaching the end of the game's storyline, however, if you have completed the side-quests throughout your travels, giving me specific key items related to them will result in me rewarding you with one of the following for each: [list]
[*] A pathway into the "secret dungeon" of the game, which you cannot get the "true ending" without completing.
[*] The option to face off against one of the game's most difficult optional bosses; myself.
[*] Entry into the game's Debug Room.[/list]
(A couple of other people had other ideas I had in mind used, so out of all my ideas I went with this one to avoid stealing other people's since they posted first.)