There is nothing wrong with you selling an item to someone who buys it with the price to be paid later on, then he sells it for however much he wants, then he pays you the price, because Allah says (interpretation of the meaning):
“Allaah has permitted trading and forbidden Riba”
This way is the most widespread form of trade. Retailers usually buy on credit, then they pay off the price after they sell the items.
But with regard to this transaction it is stipulated that the buyer should take the product from you and move it from your store or place before he sells it to someone else, because of the report narrated by al-Daaraqutni and Abu Dawood (3499) from Zayd ibn Thaabit, that the Prophet (blessings and peace of Allah be upon him) forbade selling goods where they were bought, before the merchants moved them to their places.
This hadeeth was classed as hasan by al-Albaani in Saheeh Abi Dawood.
There is no specific limit to profit according to sharee‘ah. So it is permissible for the one who buys an item from you to sell it for double the price, on condition that he does not take advantage of the purchaser’s heedlessness or ignorance of prices.
See the answer to question number 143067.
And Allah knows best.