Customers have their own budgets, which may increase over time. They'll specifically increase based on how much they like your store. This, in turn, is determined by how often you have successful exchanges with them, selling or buying.
As customers successfully purchase items from you or sell items to you, they will gain favor. You may have noticed that as you sell or buy items, you might get something called "Near Pin", or even "Just Bonus", which increases the amount of experience you get from the base 10 to 15 or 30 respectively. It also affects how much favor you get - you get significantly more favor when you get a Just Bonus, and still fair more favor with a Near Pin sale than if you just normally sell the item.
In general, regardless of their individual maxes, most customers like to buy in the 100-110% range, and sell in the 60-70% range - the exact percentage value is rolled on each transaction. If you hit the value, you get a Just Bonus. If you hit just around the value, then you'll get a Near Pin. As such, a common thing is to sell items at 104% or 105% and buy items at 65% and 66% in order to veritably guarantee a Near Pin. This doesn't really get you a lot of money, though, so this is often best done with your cheaper items to gain favor.
You do this because favor increases the budget they'll have. Whenever a customer hits a new budget level (and thus can spend more money), then after the successful transaction, a little Heart emote will show up. That's what you want to see, it means they'll start buying slightly more expensive stuff. So you need to start building up favor until their budgets rise. Then you can start selling your high quality goods.
Eventually it comes to the point that you regularly sell all your cheaper items with the intent of gaining favor, and only go for the full markup when selling the more expensive items to those who have the wallet for it.
Be on the lookout for booms, by the way. Usually you can buy an extremely cheap item for the boom, and so the number of sales you can make will significantly improve the customer favor and consequently the budgets they have.