What is the difference between buying PS from Best Buy and downloading it from adobe site?

The professional version of Photoshop is only available as a subscription, so if Photoshop is being sold at Best Buy, I think Derek is correct that is probably Photoshop Elements, a cut-down home version of Photoshop that can be sold in a box for a one-time payment.

You can subscribe to the Photographer's Plan, which includes Photoshop and Lightroom for around $10 a month, from Adobe.

You may mean Photoshop Elements (a kind of Photoshop light) which is available from various places, such as Amazon for a one-off payment of around $100.