Yes. Americans have no restrictions on what they may purchase in Cuba. However, there are restrictions on what can be brought back into the U.S. For example, you can purchase a cigar, but it must be enjoyed in Cuba and can't be brought back home to the U.S.
Due to the U.S. sanctioned embargo, the U.S. government routinely confiscates goods purchased in Cuba, which are of Cuban origin. The only exception to this regulation is informational materials such as books, films, artwork, posters, photographs and CDs. It is not permissible to bring back purchased goods, such as Cuban cigars, rum and/or coffee.
Please note that clothing and other souvenirs that normally can be purchased when traveling abroad are not typically found to the same extent in Cuba, nor is haggling common practice.