Weird sell prices when a shopkeeper likes you too much
When a shopkeeper likes you a lot (male with high charisma when the shop was generated, female with high appearance, successful hagglers, or favored race) the prices which they pay you for selling single items actually go down. However, the prices they pay you for stacks of multiple items go significantly up (i.e. you get more money from selling a stack of items than from selling each item individually). The money you get from selling a stack of items can get high enough that it's possible to make a profit by repeatedly buying and selling it.
Examples:
Female gnome selling sapphires to Waldenbrook
10 Ap: 1 sells for 749gp, 2 sell for 1498gp
17 Ap: 1 sells for 853gp, 2 sell for 1706gp
23 Ap: 1 sells for 812gp, 2 sell for 1886gp
23 Ap + max haggling: 1 sells for 574gp, 2 sell for 2296gp, 3 sell for 4005gp