Simple FAQ I did a while ago: https://pastebin.com/0Mwp3Jau Important parts are about the glitch and how to manipulate it.

For the glitch (from pastebin):

• Choose any item you don't want. The cheaper, the better;
• Move the cursor to the item you want to buy, or prepare to touch it (or both);
• Press A and/or touch the item you want;
• Before the item get to the lower right box, press X. Note that you can choose more than one item if you're fast enough. If you did it right, you'll buy the item(s) for the price of the previous.

For manipulation: pastebin may be worded a bit confusing, so: The seconds of your DS is what defines what Yue sells. It's set as soon as the game starts; this includes after a soft reset (there's a little delay between the reset and the game actually starting, so test a bit with your DS to see how much that is). That means she has a total of 60 different stocks. Also, her stock changes after you defeat the bosses. It might change after you propose too, but I don't remember right now. Also², do remember that she only sell some items after you've shipped them once. I think I did sell some stuff somewhere in the middle, so sorry if an item doesn't appear when my image shows it.

Better quality for Yue stocks (download or zoom in if too small):

• Start: https://drive.google.com/open?id=0B-vvfr4T2021THZhNlBEZ0xWWTA
• After Skelefang (desert boss, this is when Levelizer is available): https://drive.google.com/open?id=0B-vvfr4T2021NU1YdllfT0pPbVk I have 2 others, but I'm not sure when they appear. I think it's after Death Wall (ice boss) and after proposing (if it does change)?
• https://drive.google.com/open?id=0B-vvfr4T2021Rmtva3JtelFaYmM
• https://drive.google.com/open?id=0B-vvfr4T2021VVpzdmNmX3Vrb2M