Is it permissible for a sports trainer to sell dietary supplements at a higher price than the purchase price without informing the buyer of the original price, and is there a specific profit margin allowed in this case?

As for profit, there is no forbidden limit to it; rather, it depends on mutual consent between the seller and the buyer. If it is free from deception, then there is no harm in its amount.

As for receiving the price before delivering the merchandise, if the intention is to receive the price from the buyer before ordering the item online, there is no harm in that, provided that the transaction adheres to the contract of Salam (forward buying), such that the merchandise is known in terms of its type, kind, quantity, and description, is deliverable, and the full price is received at the contracting session.