Believe it or not, many Mauritanians do not know that "America" or the United States exists. An odd person may have traveled to the US or watched CNN, but that's about it. No stereotypes, no preconceived notions of wealth. No one wants something from you, because they assume you don't have more than they do. If you say you're American, they say, "Hmmm, where is that? Across the ocean, huh? That's interesting." Bedouin/Muslim hospitality is in high gear in Mauritania, and you're quickly and easily welcomed into people's homes. If more people visit Mauritania, this may change, but for now it's a welcome break from the typical experience traveling as an American across much of the globe.
A nice thing to say "thanks" is to take some photos and send them to the persons helping you after you return home. People will not accept money as a form of thanks.