Exactly brother! I don't know how high. Let's say up to 50%. I remember to read somewhere, you'd not allowed make double profit. For example: you buy something for 100$, you're not allowed to sell it for 200$ - that would be a profit of 100%. As mentioned, I am not sure if this is according to Quran and Sunnah.
One thing that we must notice, many hadith are refer to specific condition and situation, and cannot be used as exact reference for different condition and situation. Rasulullah (saw) ever said "One third, one third is enough" when a sahaba asked him about profit that he could take for his commodity. But it doesn't mean we must take 33% profit for every commodity in every condition and situation.
And it's not true if 100% profit from buying price is forbidden. In one riwayah Rasulullah (saw) gave Urwah (ra) one dinar to buy a goat. But with his skill in bidding Urwah (ra) could buy two goats and then he sold one goat to someone else in one dinar. Urwah (ra) back home with one goat and one dinar. He got profit 100% from one goat that he had sold. But Rasulullah (saw) was not angry. Even Rasulullah (saw) made du'a wish Allah give more bless to Urwah (ra).
Islamic law of trading does not govern how much profit that a seller can take, but govern about condition in taking profit which there are many factors that should be considered. If you open a restaurant in city you are allowed to take high gross profit from the cost to buy food ingredient because you need to cover many costs such as cooking cost, rent the building, employee salary, etc. But you must allow the consumer know the price before they make an order, and decide to eat in your restaurant or leave and go to another restaurant. However, if you sell raw food ingredient in village where you were the only seller, you should not take high profit.
Back to the case. Can that house owner offer and sell his house higher than the price when he bought it?. Few years ago I sold a house in price that around 300% higher than the price when I bought it 12 years before. Did I take profit too high?. No!. I bought that house in market price, and sold it in market price too. Different than when I sold my cars which always lower than the price when I bought. There are commodities which the price always increased like gold or land, there are many commodities which the price always decreased
Okay, how high he must offer and then sell his house?. The answer is, in the worth price. He can ask advice of worth price for his house from an expert. He cannot ask an Islamic teacher to definite the worth price because Islamic teacher is not the expert in this matter.
Besides, I wasn't able to get any fatawa from islamqa.com. Maybe someone else than me know any good resource for question like that.
EDIT:
PS: Statements of scholars and fatawas are more than welcome! I want to learn about the fiqh in this question with evidence from Quran and Sunnah, insha'Allah.
I am waiting for the fatwa too.