So, I began to encounter the impatient customer bug, where customers would leave immediately, but I'd previously discovered that customers will despawn inside the shop if you close the door behind them. I used this knowledge, and discovered an exploit, and hopefully I can describe it simply enough.
My strategy was to premake all items, then open shop, give them what they want, and close up immediately. Slow, but good for impatient customers. Very prone to the watcher bug, where the customer spawns and stares at you all day, but ultimately very relaxed.
So, day 13 begins, I notice two customers do 180s, and I decide to close the door behind them. I managed to do it, and the customer froze in front of the window. After offering him one of every item, he took my tin dagger and promptly walked into the wall, tossing his $122 into the chest. However, when he disappeared, he dropped the perfectly good dagger on the ground. Saving me the $100 for the tin.
TL;DR, close door behind customers, fill their request, they don't take the item with them. Profit ad infinitum?
Will test this exploit more. For...science.
EDIT: Also pointing out, customers will accept items past their time expiring, if you can catch/trap them. Also a bug.