do digital sales laws apply in the state where the product is sold or the state where the product is bought?

MOST transactions crossing state lines include a provision in the contract that EXPLICITLY places the contract under the laws of a named jurisdiction. Such provisions are usually enforced unless there is a VERY good reason to reject them.

Absent such a provision, the general rule is that if the buyer seeks out the seller, the laws of the seller's jurisdiction apply, but if the seller seeks out the buyer, the laws of the buyer's jurisdiction apply.

TECHNICALLY, the location where the product is bought IS the location where the product is bought. The issue is determining the SINGLE location where the transaction legally occurred.

Once you cross state lines it is a Federal, not a State issue.