You can download videos purchased from a model's store if they have uploaded those videos with the download permission turned on. There are two ways to check this:
1) When initially purchasing the video, you should see a disclaimer that says "Once purchased, you can download this movie in addition to streaming it on the Purchases page." Any video with this tag is downloadable!
2) If you are not sure or cannot remember, you can always check by going to Subscriptions & Purchases in the left-hand menu, under the "Store Purchases" tab along the top, and checking for the download button.