It all depends on the pricing policy of the specific seller. Ticket agencies form their own total ticket price depending on their income and expenses, and they can receive additional sales bonuses. These agencies will always make a profit, which is why they can sell tickets cheaper than the competition. If, on the contrary, the seller finds that their income is outweighed by their expenses, they may well mark up their ticket prices.
In addition, some agencies may sell tickets more expensively because they offer extra guarantees to the passenger, for example Kiwi.com.
They combine different flights into one multi‑city route and guarantee all the connections. So, in case of a force majeure situation the agency will cover all the costs of getting you to your final destination.