Charme: Beat the second dungeon and wait for her to show up in your shop. Then sell things to her for a while. (items with +1-2-3 add extra affection so use them if you've get them). She'll give you her card after a while. Also after a bit more she'll also unlock the second dungeon.
Caillou: He'll show up asking for a charred newt (You get these from goblins), Slime liver(You get this from the slime boss on dungeon level 5 in jade way or the time attack) and a bat wing(From those flying jellow/green/etc eyeballs also in jade way). Have these items on hand and he'll show up eventually.
Lancer: Find her enough times in the dungeon and she'll show up eventually in your store. Raise her affection high enough by selling to her and you'll get her card eventually. (I think I found her only in jade way so I don't know if she'll appear in the second unlocked dungeon.)
Golem girl: She showed up in my store and I've increased her affection a couple of times but she hadn't given me her card. (Help?)
Elf archer girl: I think you have to see an event where she's hurrying after her elder sister in the town square first before you can get her. She's the boss at the end of the second unlocked dungeon. (floor 30). Haven't been able to card her.
Raising your merchant level is the key to higher profits. At certain merchant levels you get higher tier items in the store/market.
If you raise the the affection of a customer enough they will show a heart after a sale. This means that they will buy with a budget that is 10 times bigger then normal. Ie girl has a 600 budget at start. Raise her heart and she has 6000 to spend. I don't know if further heart's multiply it by 10 again or just the base by for example 20.
After you've finished the main game Ie pay the last 500k you make a newgame+ save. On the last day spend all your money that is above the 500k needed. Since money doesn't go over to the newgame+ but your items/merchant level/character level does.
Also you can get the true cards from the charme/etc after finishing the game. They enable you to use the card owner instantly in the game.
Ie I got charme's card and was able to use her to finish the starters dungeon (There was still louie's dialogue though
). I also fought with charme against charme at the end of jade way
.