Does a trader have to stick to a certain profit?

There is no set limit to which a trader must adhere in the
profits he makes on his business, but if the goods have a known market
value, it is not permissible for him to cheat the buyer by selling goods
to him for more than the usual market value and taking advantage of his
ignorance. This is indicated by the words of the Prophet (peace and
blessings of Allaah be upon him): “Do not meet incoming traders and
whoever meets them and buys from them, when the trader reaches the
marketplace he has the choice (of annulling the transaction).” Narrated by
Muslim, 1519. 

Islam gives the trader the choice, when he reaches the
marketplace, because the one who met him probably cheated him and bought
from him at a price other than the market value. If he finds that this is
the case, then he may go ahead with the deal, or cancel it. The point in
this hadeeth is that it is forbidden to meet those traders on the way
because of the deceit involved and because they are not being given the
proper market price. 

The Standing Committee for Issuing Fatwas was asked: Are
profits limited in Islam, and if that is the case, what is the highest
limit? If there is no limit, how do you explain that? 

They replied:

 There is no limit to profits in trading, rather the matter
is subject to supply and demand. But it is good for the Muslim, whether he
is a trader or not, to be easy going in buying and selling, and not to
exploit the opportunity of his counterpart’s carelessness to cheat him in
buying and selling. Rather he should pay attention to the rights of his
Muslims brothers.

 From Fataawa al-Lajnah, 13/91 

They were also asked: Is it permissible for a trader to earn
more than 10% on his goods? 

They replied: 

The trader’s earnings are
not restricted to a specific amount according to sharee’ah, but it is not
permissible for a Muslim to cheat those who buy from him or to sell for
anything other than the known market value. It is prescribed for the Muslim
not to go to extremes in seeking profits but rather to be easy going when he
buys and when he sells, because the Prophet (peace and blessings of
Allaah be upon him) urged forbearance in all dealings. 

From Fataawa al-Lajnah al-Daa’imah, 13/92 

And Allaah knows best.