To take the definition of Murabaha from Wikipedia:
This concept refers to the sale of goods at a price, which includes a profit margin agreed to by both parties. The purchase and selling price, other costs, and the profit margin must be clearly stated at the time of the sale agreement. The bank is compensated for the time value of its money in the form of the profit margin. This is a fixed-income loan for the purchase of a real asset (such as real estate or a vehicle), with a fixed rate of profit determined by the profit margin. The bank is not compensated for the time value of money outside of the contracted term (i.e., the bank cannot charge additional profit on late payments); however, the asset remains as a mortgage with the bank until the default is settled.
The "profit margin" is fixed, the purchase and selling price is fixed. In other words, it's reselling something of equal value at a higher price.
The definition of Riba (from Wikipedia) is
an increase in capital without any services provided and speculation.
A definition of Riba, as provided in Sahih Muslim, Book 10, Chapter 19, Number 3846:
Nafi' reported that Ibn 'Umar told him that a person of the tribe of Laith said that Abu Sa'id al-Kludri narrated it (the above-mentioned hadith) from tile Messenger of Allah (may peace be upon him) in a narration of Qutaiba. So 'Abduliali and Nafi' went along with him, and in the hadith transmitted by Ibn Rumh (the words are) that Nafi' said: 'Abdullah (b.'Umar) went and I along with the person belonging to Banu Laith entered (the house) of Sa'id al-Khudri, and he ('Abdullah b. Umar) said: I have been informed that you say that Allah's Messenger (may peace be upon him) forbade the sale of silver with silver except in case of like for like, and sale of gold for gold except in case of like for like. Abu Sa'id pointed towards this eyes and his ears with his fingers and said: My eyes saw, and my ears listened to Allah's Messenger (may peace be upon him) saying: Do not sell gold for gold, and do not sell silver for silver except in case of like for like, and do not increase something of it upon something, and do not sell for ready money something, not present, but hand to hand.