This game is so addictive...
If you want to min-max your merchant skills, there's a great Recettear Wikia page. But if you'd like to play a bit by ear, these are good guidelines:
When people sell things to you, they like around 66% of the base price for normal white items, and 33% for blue items. If you don't care about the XP, i've rarely seen anyone refuse to sell to you at 56% or more. The little girl probably wants slightly more.
Most customers like to pay around 104% of the base price for Near/Just Pin. There are some exceptions, like the Old Man whose sweet spot is around 108%, a couple others are around 105-106%. Red items have the greatest variation here, so either ask for around 230-250% if you don't care, check the Wikia page for "exact" percents (these are a bit iffy but work sometimes).
It's possible that rep affects the amount, so if they don't know you that well, ask for a bit less than 104%.
The "pin" bonus is nice but it's just dressing for the real C-c-combo one. Does make the rep raise higher I think.
What I do once I have enough money is: always have at least 1 of every item that is white at the market/guild, 3 or more for the most basic version of each item as a reserve - don't put these on display, or even offer them unless you're out of more expensive stuff (possible exception of food which you'll probably having a ton of anyway, everyone is always selling you food and there's lots in dungeons... also don't buy treasures, maybe the cheapest ones just in case), I don't buy red items unless I'm sure I'm going to sell it right away. And stock up on blue items. Right now I try to maintain a stock of around 5 items for each blue item.
EDIT: Note that if during the "main quest" you avoid meeting Euria, she won't pester you with her scams. You'll miss one dungeon this way, but if you can live with that until the last debt payment, apparently she never returns.