There are two different ways to purchase your own prints.
Visitor shopping enabled in the gallery settings: You and your guests will see the shopping bag icon on your photos in the gallery. You can use the shopping bag icon to multi select or single select your photos. Another option is you can click on a photo to open it individually in lightbox, and from there select the buy button.
Visitor shopping disabled in the gallery settings: You can still purchase your own prints with shopping disabled in your gallery settings for your guests. However you won't see the shopping bag icon on your photos in the gallery view. Instead, click on a photo to open it in lightbox view and from there select the buy button.
You can change visitor shopping options per gallery in the Selling tab of your gallery settings.
Toggle the Visitor Shopping Cart option to ON to allow visitors to buy prints, and set it to OFF to prevent visitors from buying.
Note: Profits are only earned on visitor purchases on the Portfolio and Pro plans when a custom pricelist has been created.
For more information about selling your photos on SmugMug, check out this article.